I can say I'm issuing a fiat currency backed by gold and sell the fiat currency on Nxt's AE, but who is going to make sure that I have the gold to back the fiat currency?
Nxt could have a special kind of asset backed by fixed amount of NXT.
Doesn't matter, it's centralized decision (to provide NXT <--->, say, gold), so it can't cover whole field of risks completely (gold's holder can be robbed, can cheat and so on). Why backup so needed for minds? Money better serve its pupose with free flow, w/o any backup. And decentralized money better than centralized.