I run a forum and do a post exchange with other admins to gain posts and members to build up my new forum. Do you see any advantages to allowing people to link out from my site? Are there disadvantages other than people using it to constantly market their own stuff, like, can it hurt me in the SERPS?
The upside is that you are helping others to build links... and the downside is that lot of spammers make it just to build links....
The only disadvantage is spammy posts. But the grand majority of posts will be genuine like any forum or blog. Contrary to what _Ken says there is no evidence that too many outbound without any inbound can hurt you. If you don't have any inbound you're not going to rank period. The advantage is that it encourages posts, which in turn encourages content and more visits.
If you use it, you'll be leaking PR like a sieve. This isn't much of a problem anyway since most blog entries have few incoming links anyway.