From: http://www.vbulletin-faq.com/forum/archive/index.php/t-2013.html This isn't specific to sig links - it also stops links in posts too. But it still lets you (as an admin) post active links to sites you want Google's spiders to follow.
look at DP which has excellent rankings, they allow sig links without the nofollow tag and it doesn't seem to be affecting rankings here
You're right - I haven't seen anything definitive which says disallowing outbound links provides any SEO benefit. But it does prevent link spammers. Here's something Matt Cutts said on the topic a while back: http://www.mattcutts.com/blog/quick-comment-on-nofollow/ He doesn't mention any SEO effect, but he does mention an annoyance effect - that by allowing "anybody" to quickly and easily get links from your site, you're encouraging spammers to visit your site... and that you're better off waiting for people to get some "reputation" before you "vouch for their links". I hate link spammers - whenever I see a link on my blog which is potentially link spam, I remove it - instantly. I would hate for Google to see my site linking (ie - "giving reputation") to some crappy SEO-spam cheap-viagra-pharmacy-and-texas-hold-em-poker.com site, and give my site a big black mark. So I err on the side of caution And of the three forums I've moderated, nothing is more frustrating than deleting spam user accounts and blatant link spam. The code above allows you to limit active outbound links to certain membership groups (and "nofollow" the rest). Realistically, unless you know what the "nofollow" attribute is, and you look at the source code (ie - mainly SEO'ers) the average user won't even notice that the nofollows are active. Or that's my 2 cents