Do you use multiple hosts?

I have 20 sites so far, all on the same hosting service. They have been terrific and I have no complaints at all. What I am wondering about is that a lot of webmasters say you should distribute your sites across 2 or 3 different hosting accounts just in case something happens to one of them. This makes some sense to me but it also would add to my expenses. At what stage would you think this would be a wise idea?