Certainly on Wikipedia itself, Wikipedia (or MEDLANDS for that matter) are not acceptable sources.
I think in practice Wikitree has often accepted Wikipedia articles as a source, and I think this is not totally crazy, though it should indeed raise some concerns.
As I think there is no clear policy, I'll explain what I tend to do.
- I would only think of using Wikipedia if the article was clearly well-made and researched and had good-looking sourcing etc.
- If I could check the sourcing myself, I would do that instead. In fact, I would probably then edit both Wikipedia and Wikitree while I am looking at a group of sources.
- I would tend NOT to use Wikipedia for specifically genealogical information. Because Wikipedia is definitely not focussed on genealogy (this is even a policy, so as a Wikipedia editor I also do not add much of such information) but Wikitree is. So I would be tempted to use Wikipedia for basic historical events, like the date of a battle.
I'm sure I've broken these principles, but this would be with the idea that the article needs better sourcing in the future.