The pros, I assume, would be to include nofollow for things like Reply/Quote/Notify/Email Post buttons and other such "internal" links that search engines don't need to know about. Search engines won't follow links designated as such (though I suspect they still could find them in other ways perhaps).
As web admins here, with links in signatures, it would be expected that Google would find them, but they won't if nofollow remains in place.
The more links to your site on the web, the more importance Google gives your site (and hence its ranking) in their index...
