In other words, they're trying to be a bank! - Only holding a percentage of the actual currency they've been given by their account-holders, then presumably trading it elsewhere for commission.

This is exactly what bitcoin was designed to avoid: central control and exploitation by some self-appointed middlemen. MT Gox could vanish and overnight and a dozen exchanges will spring up to respond to demand for BTC/cash conversion services. BTC cannot be controlled and monopolised by a single entity - it's that way by design.

I've never used MT Gox and never will. They're one of many.