This was discussed before, on meta and chat.
The consensus is:
1) Since those Wikis aren't as well known, an average person should not be expected to consider them reliable even if they happen to be a top hit on Google. Being easily discoverable does not alone mean being a universal reference.
2) Being a Wiki with no citing rules, Wikias are nowhere near guaranteed to have accurate information. Harry Potter Wikias are notoriously inaccurate about canon info, mixing info from books, movies and video games frequently without specifying that (especially movies); treating movie scene interpretations as proof of canon, etc...
3) Frequently, they are quite poorly written to be a good answer to a specific question - either they are a giant mothball of info on a larger topic (with actual answer hidden in one or many pieces), or simply poorly presented to constitute an answer.
As 2 random examples, see my "E.T. and Star Wars" answer, which at least one person accused of being a Wikia copy and then the community at large realized that it wasn't anywhere near a copy of an article - it was a synthesis of many bits of info, structured to provide actual answer instead of a jumble of info. Another example is "Accio" - I had to ask a question on rules of Accio since HP Wikia article is a right sorry mess for that - it has a separate list of examples, and a short and crappy list of rules, with no linkage between the two.
Another example of crap Wiki data is a quote from my answer:
TIE Advanced x1 (which was Vader's fighter) Wookiepedia page has this to say:
The target tracking system was also more sophisticated than the already formidable advanced targeting system on standard TIE craft, used to overcome the extremely powerful electronic jamming used by all combat craft to defeat target lock in battle. For best performance, the targeting system of the x1 required frequent adjustment in combat.
... but it's completely unreferenced, and therefore probably made up on the basis of A New Hope by whoever wrote Wikia article in the first place :(