It certainly can be -- particularly if there's a lack of transparency. In the current economic climate, there's basically no choice but to keep a full reserve. And if you're holding Bitcoins, for the forseeable future there will be no choice.

But, for fiat currencies, that is really only because interest rates are so absurdly low right now. When economic conditions return to normal, it makes sense for gateways to store their fiat reserves in an ultra-low risk, interest-bearing form. It's not that difficult to structure things such that if the ultra-low risk investment somehow fails, the owners lose their capitalization but the customers do not lose their money. This should allow them to charge their customers less and also have more funds to invest in customer service, security, and features that make them more convenient to use.

I don't have a crystal ball though. My prediction of future gateway business models is basically just a guess.