The Battle For Wikipedia's Soul
njondet recommends an article at The Economist that sheds light on the identity crisis faced by Wikipedia as it is torn between two alternative futures. "'It can either strive to encompass every aspect of human knowledge, no matter how trivial; or it can adopt a more stringent editorial policy and ban articles on trivial subjects, in the hope that this will enhance its reputation as a trustworthy and credible reference source. These two conflicting visions are at the heart of a bitter struggle inside Wikipedia between 'inclusionists,' who believe that applying strict editorial criteria will dampen contributors' enthusiasm for the project, and 'deletionists' who argue that Wikipedia should be more cautious and selective about its entries."
More of a social concern about having too many articles; monitoring articles takes time, and having articles on topics that they consider worthless, but that still need to be monitored, causes the amount of eyes watching each article to decrease. This allows, in theory, more vandalism to sneak by, and decreases the average quality of Wikipedia articles, or so I've heard
You may want to read the Deletionism page on Metawiki for more info.
Me too, which is why I've started includipedia, an inclusionist fork of Wikipedia.
My thoughts exactly
There have actually been a fair number of sites that have appeared because of wikipedia/wikibooks excluding information. There have been tons of wikia sites created to hold all sorts of info that was deleted from wikipedia. And after wikibooks decided to delete half their content and ban most books about gaming, most of it was moved to other sites, like StrategyWiki or egamia. For other non-gaming related stuff, there's also (not sure if they could handle a /., but) WikiKnowledge, though they haven't gotten very big.