Double Team may be cheap. It may be dishonourable. It may be unfair. It may be looked down upon. People may not play with you if you use it.
Double Team is not cheating. Evasion clause is entirely player-enforced, not game-enforced.
EDIT GOES HERE: I'm not trying to say that it IS cheating, because it's not. I'm just trying to clarify my point.
So? I don't think anyone's going to dispute that the game doesn't have any sort of preventative measure to people using double team (after all, it's not the call of the game to define fair play); that doesn't change the fact that as you move up in scale, more and more players won't play you if you use double team, as is the general idea of a metagame.
We've long moved beyond allowing the game to decide what is fair play and what isn't; the rules determined by the players are ultimately the rules that someone hoping to play in mainstream big-leauge play will either have to abide by if he/she/it wants to get very far or better have a real good justification for why that rule shouldn't be in play (which you'd have a real hard time doing for double team, beyond arguing that it was in the game in the first place and was therefore designed to be used -- but then you're opining yourself up to a LOT of trouble).
And really, I agree with the people who don't allow double team. Much has been done by players to stamp out the luck factor in battling in general (the extremely heavy use of moves with 100% accuracy over those with even 90% accuracy comes to mind), as far as I'm concerned all double team does is bring that back to the point of brutal lopsidedness; the combination of walls and double team make luck such a massive factor that playing the game rapidly loses its fun and turns into a, well, double team-fest. Moves of the swift variety aren't justifiable as a reasonable counter as the BA on all of them, even with STAB, isn't enough to put a dent in a wall, and buffing would simply play into the hands of said wall.
Also Umbreon? Taunting gyradoses, y halo thar.
As far as I'm concerned the justification for banning double team is simillar to that of banning ubers -- among all the things that make the game lopsided, they make the game TOO lopsided and require the player to abuse the strategy in question (ubers on ubers, double team on double, team, etc.) in order to counter it.