Our basement would be completely dry if it wasn't for problems with the driveway. I'm also guessing that either the outside isn't sealed or the sealant has failed. Even with the rain this spring, we didn't get any flooding.
I don't intend to ever buy a house where a sump pump is the only hope for keeping a basement dry. For one, when do you suppose the power is most likely to go out? On a sunny day or a during a heavy storm?
Plenty of dry basements are damp and musty though. Basements just tend to be humid and a lot are not sealed well. The "dry" air inside the house simply draws moisture from the earth. Anything that stops or slows the that flow helps, whether a vapor barrier or sealant. I'm sure it doesn't help that a lot of unfinished basements don't have a cold air intake, so air movement is minimal and the dampness isn't vented out.