I could definitely do with a few Dolphins visiting my blog, although to be fair once they get there all you ever see are resteems anyway! Always a good selection of posts to upvote though.
Comments help me a great deal to keep track of people, although I use the comments on over people's posts even more then on my own. In fact sometimes the comments on a post can be even more entertaining then the actual post they are attached to!
Comments are actually a great potential for earning as well, a well made comment can get very good upvote value - I know quite a few people who end up getting more from comments then posting.
Not everyone rewards comments, but then again if you are not at least half a dolphin it takes too much of your VP to do so. I would say this statement cant be very true for the small fish.
I think it is kind of a gold mining of comments idea, if you spend all your time leaving well thought out comments on whale and dolphin posts then chances are you are going to see quite a few decent comment upvotes. If you are like me, and spend most of your time supporting and encouraging redfish and minnows then no you won't see much in rewards. I do now and again tidy up other peoples blog comments by upvoting the good ones to move them to the top, sometimes it can be luck more than judgement.
I think for redfish and minnows getting their posts seen and appreciated is still very difficult, where a good comment on the right post will get seen and rewarded well.