...That's the case with ANY link. If you don't build the quality of the forum profile itself, why would you benefit from it? It's a deep page with no links.
Forum profiles are like any other webpage, if you have 100 PR0 links that are only indexed... What do you expect to come of them? You aren't leeching any PR from the forum itself, so even if it's PR5, your profile is still PR0.
You aren't building any links to it, except for no PR RSS links. So where is the PR coming from to make this forum profile a quality page to get a link from?
Forum profiles are entirely controllable by you as far as content and outbound link density. How is that useless? Blog comments are (not only nofollow) saturated with shit links and content all around you, you have no control over what the POST says or what links are in it. You can fill a forum profile with SEO content about your keyword and DF anchor link out.
How is that useless?
If you utilize them correctly, I think you'll see a difference in the quality of your links.