Luckily, so far none of these “forks” of Wikipedia has tried to compete with Wikipedia by building its own user base or evolving its own content, so user effort remains unified. Putting ads on Wikipedia might blur the line between Wikipedia and sites that syndicate its content; it also might encourage a group of disgruntled Wikipedians to take the existing Wikipedia content and continue to develop it elsewhere, perhaps on a new ad-free website.

This possibility of a major fork of Wikipedia is admittedly very remote right now. But it might become more of a concern in the future as Wikipedia becomes more influential and people begin tugging it in different directions.