Exactly. Ripping a zipper open isn't terribly hard. Slashing the bag is even easier if it's a softside. And if they really want the bag they'll take the whole thing anyway and won't bother to try and unlock it. That's always been the case. If a thief wants something, they will find a way to take it.

I use locks to more or less keep the bag from popping open or accidentally being ripped open, much as Randyk47 said.