Where as without RMT competition (ie Arena, Ladders, etc) is strictly dictated by the game as is, with RMT competition is dictated by the game and economics simultaneously. This is considered destructive, of course, because the game-play as is corrupted ( To taint; contaminate ) by external forces. Corrupted, of course is also defined as "To destroy or subvert the honesty or integrity of", thus, RMT is destructive.
But AH is dictated by the game itself...
Any item you see in the AH must've been found in the game to begin with. So essentially you bring up theat trading hurts the gameplay (then we can argue if someone buying their gear actually hurts your experience or not). But infact, trading is a core mechanic in the diablo franchise because of the low drop chances on certain items.
Anyone can buy themselves powerfull, in any game, if it includes trading... but now for the first time it's done with a positive edge to it, that it is all player regulated and player to player and it's also mcuh safer since you can't scam.
The destructive part you are talking about exists with or without RMAH. But now RMAH brings positive stuff aswell so the general sum of that is that it's a positive change. The only way to fight these destructive parts will be to remove trading. And I hope you agree that is a very very bad idea to the gameplay experience.
Q u o t e: It's wrong because blizzard and the general community knows it hurts gameplay and the legit player
But allowing it and have a game designed with that feature built in won't hurt it... the reason why it hurts gameplay is because games never liked it or had it... but people circumvented that and destroyed the gameplay.
And I hope that it will be like WoW where you can tell on how they played which bought accounts or bought power... because they played like crap and were still successfull. So you will still be the better player.
But we also don't know this untill we try the game, so saying a player who buys gear without a care in the world will have an advantage over the player who cares and learns how to play and get just little less good gear isn't a fact. Nor is the opposite a fact and it's pointless to have that as an argument to the matter in the first place.