you should also be worried about one BTC-USD exchange being 90% of the bitcoin exchange.
If MtGox goes down, even temporarily, it could have severe repercussions for people's faith in the ability to transfer between BTC-USD.
It also would be an enticement for another exchange to be created, given the profits that can be had being the facilitator of exchange. I think someone would put up on pretty fast, just to get a cut from the pie. Even if an exchange didn't exist, bitcoin still has value between participants. The real effort should go into getting merchants to accept bitcoin directly, honestly.
I don't mind an exchange being there. Just thinking down the road where such exchanges may not need to exist.