I would like to ask you about the signature in those forums in which neither "[url=..." nor "<a href..." links are allowed in signatures. My questions is: Is it still any useful/worh it for SEO purposes to include a websites' url in these forum's signature outside of a link?
If you make intelligent posts in a forum that's relevant to your type of business, some users would copy and paste a signature like that to check out your site. It could have value for direct traffic.
But don't count on it for SEO value. We know that the search engines try to give less weight to links that are self-generated. We don't know what kind of weight that unlinked "citations" are given, but it would be reasonable to expect that they'd get similar down-weighting if the search engines suspect they are self-generated.
Assume that such a forum signature could be useful for reaching targeted human visitors, but very little use for SEO.