I have seen a few answers that use Wikipedia as a reference by saying words like: "according to Wikipedia", "Wikipedia says that" and etc. As you already know, everyone is free to edit topics in Wikipedia and this kind of freedom may open a door for many misconceptions. Now my question is that how reliable Wikipedia is when it is referred to? I am asking this question because as a beginner when I read a topic in Wikipedia, I am not sure about its healthiness.
I would say that Wiki links are acceptable for basic definitions or a general overview of some topic, especially if there is no freely accessible alternative. Ideally, the answerer should have some expertise in the area of the question and before linking to Wiki can verify that the article is of acceptable quality. I do not think that Wiki articles on technical subjects are likely to deteriorate over time, it's more likely that on some more obscure topics a decent entry was never written.
However, I would say that an online copy of a book is always better for definitions, and for all results the primary reference should be cited, maybe with Wiki as an additional link.
Finally, as Artem pointed out, we expect that almost all Wiki citations in CSTheory would be no more than supplementary reading: if a question can be answered by a Wiki reference, it is unlikely that it was a research level question.