Even then it would be rather pointless, sure you could get them in the air, but there just woulden't be any room to fly around in, so you'd allways fly around in large circles, and not have the room to do propper realistic dog fighting (and besides, how's that helping your team win the map anyway?).
This is the same problem BF1942 had, the planes just felt like an afterthought, something that was put in there just for it's own sake, just because they could, and whilst players may have had some fun with them, moreso as BF1942 never did aim for the same level of realism as RO, they ultimately didn't really serve any practical purpose, they where just for the lulz really.
In a game of the scale of WWII-Online, planes have a purpose and a place, but in the much smaller scale of RO and the BF series, i find them to be pointless fluff, all for it's own sake, devoid of realism or any real purpose, and for the sake of keeping RO both fairly realistic and upholding a coherent objective based mode of play, i agree with TWI's reasons for keeping them out of the game, it just could not be done good enough on this scale, it could not be done as realistic and nice as the Tanks, and would feel out of place.
Some things just aren't worth doing unless you can do them justice, and i'd say this is one of them.