Lawrence M. Sanger, who co-founded Wikipedia and Citizendium, has published an article which explains perfectly why I don’t usually bother contributing to Wikipedia (p. 64):
Over the long term, the quality of a given Wikipedia article will do a random walk around the highest level of quality permitted by the most persistent and aggressive people who follow an article.
The difficulty, as many disaffected Wikipedians have discovered, is that there are far too many articles persistently “managed” by aggressive individuals who will simply not let it improve in certain respects. In disputes, these persons tend to drive off more knowledgeable people, thereby keeping the quality of articles low.
Yep, that’s it. I’ve seen this phenomenon play out several times. Someone who is factually and demonstrably wrong but passionate about a Wikipedia page will edit out anything which does not correspond to his incorrect views. There have been very many times I decided to not correct Wikipedia pages because I just do not have the luxury of wasting time. If I knew the improvements I make would not be undone by someone who obviously does not know what he is talking about, I would spend time editing because the time thus spent would not be wasted. I’m just not interested in spending precious time writing thoughtfully about a subject to then have some random guy who thinks he knows better mess it up or delete it entirely. Wikipedia apologists will say that it is always possible to discuss the edit in the Talk page. True, but that does not solve the problem. It makes it worse because it means trying to convince someone who has given up on critical thinking that his opinion is incorrect. This means wasting even more time. So I’ll leave it to other people to participate in the random walk. My view on the topic can be summarized as:
Editing Wikipedia is a waste of time.
You have to take that statement in context. It means that the time I could spend on Wikipedia is better spent elsewhere. There is always something to write elsewhere or something to read or some software to improve or something else I can do with my time which will be more fruitful than engaging in a futile edit war with some ill-informed guy.