A good thing about the links is that you might reply to a good post and that post will get many backlinks to it which in turn passes pr to your site!
Ya I am agree that forum signatures useful for Search Engine Optimize service. It's helpful to SEO & SEM Services and also Ecommerce solutions.
Signature links in do follow forums always help, be it in helping rankings in SERP's or building backlinks they are always useful.
If forum is related to your niche then it's definitely help to get ranking and of course it will give you back links as well.