Pretty much, yes. You'll get no googley goodness at all from a "nofollow" link. The only benefit you'll get is the small chance of a little bit of direct traffic from fellow readers if your link is very relevant to the discussion and looks interesting and you build up a reputation as someone trustworthy with something to say worth listening to.
Forums aren't here to help you build up your link profile, they're here to build communities of like-minded people discussing topics that interest them. If you want to join in one those communities, that's great ... and you might get some benefit to your site on the side. But if you're only in it for what you can get out of it, don't bother, don't waste your time and everyone else's.