For quite a long time, I've been meaning to write a summary of the key
reasons not to use "non-commercial uses only" licenses. It is now online at:
http://intelligentdesigns.net/Licenses/NC
I posted it to Kuro5hin as well, where it is now being discussed:
http://www.kuro5hin.org/story/2005/9/11/16331/0655
The version on the wiki is slightly more comprehensive and will
hopefully evolve over time.
This is not specific to Wikimedia, but I hope that those Wikimedians
lobbying individuals, governments, and educational or scientific
institutions to put their content under truly free licenses or into the
public domain will find this collection of arguments useful.
Best,
Erik