Maybe you have seen the buzz out there about blog spam commenting.
Spam comments are those that do little to add good ideas to the discussion. What is the point of posting "Nice article!" or "Good job!" unless it is just to get brownie points with the person who wrote the blog? I guess you could make the argument that you were just in a hurry and wanted to show the blogger that you cared.
Well, I'm not buying that. I think you are more likely, in that case, to be trying to get someone to click to your website or to get some google juice.
Another issue I have with blog comments is that there is not much social interaction going on if the commentors are not talking to each other, and not just to the blogger. Here is an example of a blogger who gets involved and obviously (and successfully) gets some interaction among the posters: http://johnhaydon.com/2009/08/thesis-headway-war/#comment-14574581
I think there is much to be gained by reading good comments. I do it all the time. More often, I click on the commentor's name to see what he or she is doing on their blog. To me that is great value as so much of what I read in comments is just a lot of bruhaha.
Another problem I have with comments is that they are just too long. I mean, you should by now be able to pack a lot of punch in 140 characters. (ok, maybe a couple of paragraphs) But to blather on and on, give me a break. You aren't that smart. (and neither am I)
End of rant. Have a great day!