The thing is, Wikipedia usually has decent articles on well established stuff, like explanations of the Theory of Evolution, Relativity or similar, and it can serve as a fair introduction to those not familiar with said topics.
I'm not terribly surprised if articles about, say, Sarah Palin sometimes falls victim to pranksters and vandalism, seeing as she sort of put herself up to be pranked in oh so many ways, although, of course, as someone who believes information should be as accurate as possible, I don't condone such things.
Usually though, such articles get closed down rather quickly and are removed from public editing, so it's not a lasting problem.