That's right, but it would increase your cost of litigation substantially, since you'll have a much longer court case trying to prove that.

It's not something that you really want to rely on.

edit: Just an addendum - most of the deadbeat weasel clients know that verbal contracts don't carry a lot of weight, so the threat of being able to sue is really the important part. (Ideally you refuse the deadbeats upfront, but it's hard to tell sometimes)