it all depends on where you are commenting. some places still do not have the nofollow tag so it would be useful there, but most of them these days have the nofollow enabled. if you post something relevant to the topic you may get a click from someone, but it wont help you with backlinks if nofollow is there. best bet is to just comment on places that do not have nofollow. there are a few left, but not too many so get it while they are still there