The actual definition of benefit is what we need to understand when looking at commenting on blogs.
Let me share with you few examples and choose which blog you would prefer to be part of and comment on.
- An authority blog in the related niche where you don't get a dofollow link but the link is a nofollow.
- A new blog in the related niche which offers a dofollow links to get comments on the site. This actually creates lots of comments on the blog but only for getting the links.
Now if you opt for option 1 you may not benefit entirely for being linked from that blog but actually you are leaving your link for actual audience on the site and if people find the comment useful they will end up on your not as browser but as reader.
If you opt for option 2 Google would definitely see your link but your link will be in the midst of thousand other links and so the actual benefit of being dofollow link is lost somewhere.
So now what benefits you are looking for is the REAL Question.
Blog commenting only for links is never ever beneficial according to me. I am yet to see a site which ranks pretty much for good keywords only with links from blog comments.