@wacky - both are important.
Active forum members help contribute, bring new content/guides/screenshots/etc, new wiki contributions, and new people to talk to.
New lurkers increase ad impressions and maybe one day the money we get off ads will actually *gasp* pay for the server costs ($0.00 total ad revenue received to date YAY!)
Restricting the wiki for viewing is a very bad thing. Having the guides restricted like MT did increases the number of lurker members, but doesn't increase the number of active members one bit. If people want to post, they'll register regardless of the sites' viewing restrictions - and restricting wiki/guides/etc makes it so search engines cannot index them.