Hiding sigs could deter spam? I thought of the exact same thing too. I mean if spammers could see that we allow signature link promotions they would want to target our site even more.
I've allowed guests to see signatures because there is one member on my forum who relies on signature to index all his posts. If the benefits for hiding signatures were so compelling then I'll disable it but he will not be happy with that.
It's this video below by Matt Cutts which made me think about forum signatures.
Hiding signatures for guests does have some SEO value actually because of how PageRank works. Every nofollow link that the search engine spider sees, the less PageRank you have flowing to your normal links (nofollow links more or less devalue the worth of "dofollow" links on the same page).
So what happens when you have a page with “ten PageRank points” and ten outgoing links, and five of those links are nofollowed? Let’s leave aside the decay factor to focus on the core part of the question. Originally, the five links without nofollow would have flowed two points of PageRank each (in essence, the nofollowed links didn’t count toward the denominator when dividing PageRank by the outdegree of the page). More than a year ago, Google changed how the PageRank flows so that the five links without nofollow would flow one point of PageRank each.