Ok, I'll offer a little explanation as to how it works.

First, the importance of the link depends on various factors. I will list these factors in order of importance:

1) The PageRank of the page on which the link is placed.
2) How many other links are on the page.
3) Where on the page the link is located (higher up = more value).
4) How related is the information on the page to the information on the page of the link.

Your average forum post has a very low pagerank therefore the link you add is worth very little based on that factor alone.

Multiple links from the same site definitely are not worth as much as multiple links from different sites (assuming all conditions are equal).

Having said all of this, there is an example where posting multiple times will yield tremedous benefit. This is when your forum profile has a link.

I have seen posters on some forums with thousands of posts and when you look at their profile page it has a high pagerank. Then there is just 1 link on their profile page - boom, huge benefit.

So you can actually get a good link from a forum even without signature links as long as the profiles are linked to and as long as you can have a link on your profile.

Another thing you have to be careful with these days is not to have all your links use the same tags. For example if all your links use "Best Seo Company" in the text of the link Google will pick up on this. Be sure to diversify.

Hope this helps clarify.