I'm sure most of them do have, but some can be much worse then they look.
When I picked up my car there was no visual signs of rust and it didn't look to bad underneath. Yet when I stripped the kit off it and started poking around it was a very different story.
Then my mate bought one, with more miles on it and it was solid as a rock in comparison.

That's why you need to have a real good look round and under the car, and if you do see anything bad take into account the work you can do yourself or if your willing to spend etc.