It is a good idea, but you need to establish policies for your forum. Many people just join forums to post their link as some have already said. We have a policy that any fluff posts in our forums will be deleted. If we determine that someone is posting junk just to get their signature seen, we boot them from the site.
agreed thats part of the spam world and many try to take advantage that way.... not worth it though.... I use my forum to communicate with my clients and advertise our products and service ......basically it depends on what your marketing... connect your forum to your domain and build your client base...
as long as you limit the signatures and be strict about it, it will be fine. Don't allow increased font sizes or annoying colors, just 1 link.
you should make it so they can have a sig link once they have a x number of posts or have been members for a certian number of days. That way they will stay long enough to get the sig space
I like forums that let me show off my amazing banners! I say start it off at 1 or 2 lines then you should make it a 1 time fee and then the person can have up to 1 banner in their sig, but htey don't have to buy if they don't want.