This question has the curious effect of calling into question a presumption we all somehow harbor: that Wikipedia is a source we may, by and large, safely believe and rely upon “for most intents and purposes.” But why do we actually believe Wikipedia?
An anecdote illustrates this issue: When I recently asked a graduating doctoral student of mathematics about some rather obscure concept in his narrow area of specialty, he told me to look it up on Wikipedia. Then he added, as a supplemental explanation: “If it is on Wikipedia, it is probably right.” And, “Just consider the kind of person that is likely to publish on Wikipedia: it must be a nerd who really cares deeply about the subject to write an extensive article, and so he probably does know a lot about it.”
This is one way of looking at it: a volunteer putting in several hours to selflessly write an anonymous article must really care about the subject matter, and must be passionate enough about it to want to educate the general public. Dedicated enthusiasts do tend to know their area of interest rather well.
Another basis for our trust in Wikipedia as a source is the consistency of knowledge provided by this free encyclopedia: We virtually never hear of experiences when someone would claim that an article on Wikipedia contained an outright falsehood. Sins of omission, fuzzy around the fringes, not enough attention given to minority views – sure, all that cannot be ruled out, and typically it is not. All those ultimately inevitable limitations of any encyclopedia would predictably attach to Wikipedia as well – but it is right here at your fingertips, at the beck and call of the digital age. If it was approximately right in all the previous cases of our use of it as a reference tool, it will probably be equally accurate in future cases. Considering past track record in quality control is one of the ways we come to trust our sources.
But there is also the issue of a quality and peer review: Wikipedia articles can be edited, questioned, footnoted, cited, cross-referenced, and amended. Somehow people seem much more compelled to object to the errors of others and point them out than to contribute a more accurate substantive text by themselves. This invisible network of reviewers consisting of the public at large, or actually of other enthusiasts who care about the same subject to a similar degree and who are knowledgeable enough to contribute to its accurate description, is one empirically meritorious way of ascertaining that the community of specialists keeps satisfactory control of quality issues in Wikipedia articles. No, we cannot rule out with any satisfactory certainty that a relatively recently posted article that has yet to undergo the multiple grinding of peer review may still contain spin or be reflective of some kind of special interest, but sunlight – and democracy – are generally fairly reliable disinfectants.
So, why do we believe Wikipedia, other than just by default? I think we may safely invoke for most purposes the standard of “general acceptance” – the one extensively discussed in Frye v. United States, 293 F. 1013 (D.C. Cir. 1923) and in Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993).