Anyone who has their own blog can remember the feeling u get when u press that button to submit and make it viewable to millions. ...You could have spent hours researching , time sifting thru copyable ads and adapting them to show ur individuality...or spent hours on end tweaking your html code to improve the numer of visitors to it.

So when you check in the next day it is lovely to see a comment to ur post...a sense of achievent...someone has taken notice of ur efforts and they offer a recognition for it.

Kind of a praise and reward encouagement for you to start to work on your next post

I consider a signup as a better comment than a comment is, but you are right in saying that it pays to make actual comments. It can pay off in all the ways you mention and a few more besides.
Any time someone leaves a comment on my blog site 999 times out of 10,000 it will repeat exactly what I said, then sign their names. That is not a comment. Nor will it ever get posted on my site. I don't care if you are for my subject or against it, always say something you actually mean, or don't say anything at all.

Good post on blog interaction, Shri.

I enjoyed it.

Asher