"nofollow' links are far from useless to search engines, or to websites - read the wikipedia article
nofollow - Wikipedia, the free encyclopedia
It's a badly named link attribute, patented by Google employees, but used to varying extents by some of the other search majors. What it means is "exclude link from ranking algorithm". The search engines may still follow (which is what makes it badly named, in my mind) the link, to discover new content. So spamming links to your blog articles may be a good way to get the search engines to discover your new content. Not that any of us would do such a thing, of course.
WMM uses nofollow on its pages, but maybe vBulletin's authors are confused? If you look at the Page Source for this page (you do this all the time, of course!) you can see "nofollow" used for lots of internal links (not the ones in our sigs or posts) - that's completely redundant. According to the wikipedia article, if you really want "don't follow this link" functionality, you need a 'nofollow' value in your robots html META element.
Even if robots don't follow them nor search engines rank them, links are still good if they bring visitors. Everybody watch their stats to see which links bring the most visitors?