There are forums within certain niches that allow links within signature lines.
Some forums use "nofollow, no index" meta tags. However, I've found that even links from those forums show up if I do a link:www.mysite.com search.
The problem is that any links from any one site begin to diminish in value the more often they're repeated.
For example, there's a forum I visit at least twice a day, and have been for nearly nine years. It's related to the niche of one of my sites. If I do a link:www.mysite.com search, I'll find literally thousands of links from posts I've written on the site.
Are the links from those thousands of posts worth as much as a single link from another authority site within my niche?
I don't think so. The value of links from a single site diminishes. One is probably worth more than 4,000.
Do I know this to be absolutely true? No. But I can only surmise that it would be too easy for someone to put a link in a sig line, and then submit dozens of worthless posts every night.
I view the links coming from forums I regularly post on as a little bit of gravy. But I spend much more time cultivating single links from well-respected sites within my niche.