In my opinion, if not be deleted, that comment is useful.

I think not how much
but you have to look for the related site

All comments help in one way or the other.

On blogs that have "dofollow" for their comments, it's a backlink.

On blogs that don't have "dofollow," you're still introducing your link to the audience of that blog. However, for anyone to care enough to actually click on your link, you'll have to make good, quality comments (something you should always do no matter what).

I've been added to the blogroll on some blogs just because of the relationship I built with my comments.

As long as you can find a blog that you think is useful.. However, if that takes too much time consuming then I suggest you don't focus on it much.. move on with your other task...