There are pro's and cons.
First of all what makes a quality link?
1. Relevance, Relevance, Relevance
2. links to page/site
3. quality of those links
4. the quality of those sites back links
5. do these sites that link to the page your link is on rank for anything
6. age of site
7. sites cached
8. sites indexed
9. IP farm
10. sites content
There are many others but those are some major ones
With forum posts IMO you dont get a lot of what is required for a quality link.
forum sigs are posted on pages that are dynamically generated and might not be cached, have little to no back links, normally horrible keyword density and keyword proxemity.
I have no idea if a search engine devalues a link based on the fact that its a forum link but probably looks at the page more then anything.
Also with google doing so many hand jobs they could
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