Air units are a source of tremendous cognitive dissonance in RTS games. As a metaphor, air units usually seem awkward. They hover in the air infinitely or, if the developer wants to be "realistic", they fly to accomplish some objective and then run out of fuel and ammo and have to fly back and land again. The sorties usually do not see units going too far afield, which makes sense considering air units that would realistically go 100x the speed of ground units instead travel at barely double their speed (at most). In terms of game mechanics, seldom do air units make sense and offer balanced viable options for a player. Air units often are the most powerful units in the game (Battlecruisers and Carriers in Starcraft, bombers that can level base buildings in one run in RUSE) if they are allowed to be built on-map and are treated as units.
Land-based RTSes are generally based on map control. The map is critical to how the game unfolds. Where are resources? Where do players start? Where are the impassable boundaries the players have to work around? All of this is circumvented by air units. Air units generally do not exert map control unless they're implemented simply as ground units that ignore terrain. Ignoring terrain is, itself, an issue in games where much of the interesting strategic choice blossoms from terrain.
When given the viable option at the beginning of a match, a player should almost always choose air units before they begin to use ground units to cement map control. An air unit that is equally as effective as a ground unit at ground attack is significantly more valuable in that it can ignore terrain to harass the opponent from any angle. Since games have a sharp divide between units that can shoot air units and units that cannot, the early game units generally are putrid at air defense. If they were good at air defense, then air would never be a viable strategy because building basic units would hard-counter it.
Most standard RTSes circumvent this problem by requiring a bit of tech research before the player can buy air units, or by nerfing air units to the point where they are weak enough to not be much of a threat unless massed. Both approaches remove air from viability in the early-game. The best approach to air unit design allows air to be effective and viable throughout the entire game--or at least until the opponent builds counters.
Air units are too fast and too long-ranged to be presented effectively with similar mechanics to ground units in most RTSes. Unless the game is on a very broad scale--a scale which is very rarely attempted in RTSes--air units will not fit into the balance of the game. The speed of air units can cause then to be a must-have in the early game because they can project force much farther and much faster than any other unit and then run away from danger just as quickly. The advantages of going air may be too great for competitive palyers to pass them up, as they were in RUSE during open beta, which leads to the set of viable builds being constricted because the player needs to build air (or a significant amount of ground-based anti-air) first.
There are two ways to "fix" air units in land-based RTSes.
The easier but less satisfying way involves making all air units act as if they're nothing more than ground units that ignore terrain. These air units have to have speed comparable to land units, or perhaps be slower, to avoid obsoleting ground units.
The best way to solve this problem (at least that I've encountered) is to make all manifestations of air power into special abilities. "Off-map" air. Company of Heroes does this to great effect. The key is to not make on-map anti-air units required or common. Give otherwise-useful units the ability to shoot down planes if the planes take certain paths. For instance, the flak 88 in Company of Heroes is a powerful, long-range anti-tank gun primarily, but also acts as a supremely powerful anti-air gun that can shoot down a plane in one volley. As long as air use is relatively rare in the context of the game, making all air units off-map call-ins tremendously increases the seeming realism and fun of air units while doing nothing to damage the metagame.
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Total Annihilation (and by extension, Supreme Commander) tried to resolve the problem by making air units fast and fragile and air defense inaccurate and plentiful. The default AI of air units caused them to align themselves with the target, speed towards it, fire while flying past it and looping around for an another run. Because this behavior required a lot of space, any plane attacking a base was bound to take at least some damage if the base had any air defense at all. In other words, air defense was meant as a soft counter: Air units were still useful, but equal-value air defense would eventually prevail.
Unfortunately, the designers forgot to implement collisions/collision avoidance for aircraft. For land/water units, group size quickly becomes a restraining factor. 100 tanks is only going to cause a traffic jam (especially if there's a chokepoint), leaving the tanks at the rear unable to fire at the enemy. By comparison, all 100 planes would be able to take a shot at the enemy, usually killing it with alpha damage alone. Crafty players could even force the aircraft to stack on top of each other, minimizing the group's footprint and thus vulnerability to air defense.
I would love to see an implementation where having more than X air units in an area would incur diminishing returns: The pilots would either have difficulties clearing a path for a bombing run and thus reduce their DPS, or would just crash into other planes or even terrain.
RTSs are more abstract strategy than they are war simulator. Air units are simply units that can bypass terrain. But that does not mean they are uninteresting or strategically defunct.
StarCraft does 'abstract war game' really well. Look at a competitive StarCraft or SC2 replay/shoutcast. You'll see air units get used as main unit composition, surprise harassment, or counters. Sometimes they are only viable for a small timeframe (using Corsairs to kill Overlords in SC), and when the opponent catches up (Hydralisks), the player transitions to something else (High Templar). As you say, they are really just ground units with special abilities.
Realistic air artillery strikes are not needed to make SC(2) a fun and competitive game.
Mot, I didn't say that there can't be air units in a competitive RTS. I'm just outlining how to keep air an interesting strategic possibility in a land-based RTSes without basically turning it into ground units that ignore terrain. Starcraft games have always been in the "ground units that ignore terrain" shop--I think that style is much less interesting than the CoH off-map call-in style.
I like how the Hearts of Iron series treats air units. By themselves, they are a mere nuisance (unless the enemy tech level is vastly higher than your own; say with Mongolia trying to fight Nazi Germany after your big brother Soviet Union already fell ... but then you have way bigger problems than just air superiority). They also require some level of concentration - mostly along the lines of naval air power (naval bombers and carrier air groups), strategical bombers and tactical bombers, with a healthy mix of fighter or interceptor power mixed in. However, used in combination with other factors, they can turn the tide quite easily.
Naval air power reins supreme (as it does in real life), but is a late-game tech and requires heavy investment; also, the carriers by themselves are sitting ducks without proper support ships. Strategic bombing can cripple your enemy's economy, but is a long term strategy and requires a good defensible position for your country; not every nation has the time and possibility to use it (Great Britain being the prime example, of course). Tactical air power is a great addition to land based combat, turning even battles into easy wins, shielding your own troop's movements while making enemy troop's deployment and - especially - retreat a costly affair. Then there are the paratroopers of course, which can be a mixed bag (as real-life operations Mercury and Market Garden showed).
hearts of Iron is on an appropriate scale for 'realistic' air units to make sense, so this post doesn't particularly apply to it.
My opinion.
I might have gotten carried away...
I know I'm really late to the party but... What about helicopters? they fly at a reasonable speed usually. Have the ability to hover. The only thing I could see wrong with them is that in some games (CnC Generals) They re-arm missiles in the air and have infinite cannon ammo. But other than that... I like air power, but I would like to see it changed slightly, To make units more useful. Such as air superiority units, you never seem to need them...
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