Nasri can use both feet extremely well and i think he can be just as effective on the right as on the left. Same as Arshavin really.
I have no idea why he's bad on the right, but theres a reason he's barely played there, and a reason why hell, even Walcott was played on the right instead of Nasri when Arshavin was on the left. I can't tell you why, some players are just horrible on one wing, I honestly can't remember what game it was but he messed up every pass, cross, run, dribble.
I don't see the problem, people practice in one position for years, and years and years. Nasri even said he'd be happy playing DM and didn't mention prefering to play on the right.
I feel sorry for him, the way the squads shaping up he's never going to be first team, Arshavin just came and blew him out of the water, he's just way to good to not be played whenever possible. Though personally I'd play Nasri left, Arshavin right and no Theo, instead of Arshavin left/Theo right.
Problem is Arshavin is best through the middle and works best on the left if he "has" to be on the wing because his game is cutting in and crossing/shooting with his right. We've got a squad of loads of fantastic players but far to many general central attackers being pushed out of position. We've got about 10 guys who could play central attacking mid or withdrawn striker, its getting ridiculous.
EDIT:- no, he's not fast enough for the wing to be a truly world class out and out player there, he can be good, just not brilliant there. He "does" a player but can't pull away from them like Ronaldo or a whole host of other players could, one of the fastest at the club, not even close tbh. If you compare a Nasri to a Young/Malouda/Ronaldo, frankly, he's very slow compared to them, its why his best games have all been right behind the main striker. Those games post the 4 draws he played withdrawn striker and the team was fantastic, clicked perfectly and he was man of the match. In two of those games he was annonymous in the first half on the wing, pushed into the middle and was the best player on the pitch. Theres an incredibly clear difference between his effectiveness in those two positions.