Namespaces are usually a pain in the ass. Sj's post on the subject: https://meta.wikimedia.org/w/index.php?title=Talk:Wikimedia.org&diff=365735 6&oldid=3657294#Separating_the_wheat_from_the_chaff.
From his post: "... but that would address many of the reasons that people fragment what are otherwise sensible critical-mass communities or projects into multiple beautiful-but-subcritical communities which fade over time."
I don't think we need namespaces. I definitely don't think we need multiple wikis, though. The best part about a wiki is that if we figure out we did it wrong, we can change it.
- Ryan