In any case, having rel="nofollow" in a link means that those pages that are linked to do not get any bump to their page rank from that link. Some people write comments specifically to generate traffic and page rank for their own pages. It's a symbiotic relationship, since generating content for your page will increase your page rank, while their generating links to their own pages will increase theirs.
Allowing forum signature links, and profile links from comments to generate page rank for those who have them would give additional incentive for others to post in the public areas of the site. It would also increase the chance of junk posting, and cause you to loose some control over what kinds of sites elliquiy is associated with (though of course, due to it's association with sites with links -to- elliquiy-, such control is largely an illusion anyway).
I'd like to point out that until I clicked on the To-Do list out of curiosity, I wasn't even aware that there were blogs on the site. If you want people to comment, it might be a good idea to make sure people know there are blogs. I just kind of ignore anything I'm not specifically looking for on the big bar at the top. For example: I've never taken a second glance at the Wiki, because Wiki's just aren't my thing. Maybe a little bar below the To-Do list?