The theoretical implementation of Blockchain means that they are unable to actually access your private keys and steal your funds. If you are ever worried you can still import into Multibit anyways.

Mt Gox was big because it was one of the earliest players and a lack of competition didn't help that issue at all. Once we see more and more competition for services willing to exchange, accept, pay in BTC we will see less and less major scams as each business (to call it that) would take progressively less market share. As well as that having a slice of the market would one day be a lot more profitable than trying to scam users, plus you wouldn't have to worry about being hunted down.