Do backlinks from signatures at boards help any or does it only help getting your site indexed more? Is that the only positive effect it has?
They help a bit, more so if your links are from a forum which is directly related to the topic of your website.
Forum signature helps in: 1. Attracting more traffic to the site. 2. Counting as backlinks to the site. 3. Allows faster indexing of sites by search engines on popular forums.
I would not place such a high value on links from forums. I believe the value from them would be in the form of your website being constantly being spidered. Since you could build thousands of forum links in a short amount of time I think that the SERP's would see that your links come from the same IP address and start discounting some of them. There is no quick way to cheat the system that these search engines have created. We are always developing programs that counteract their new algo's. It's not an easy business but once you understand what they are trying to do it gets a lot easier. I have a couple friends in Google and even though we are really on opposite teams we both understand why we both have jobs.