Here is my problem with do follow blogs. The number one reason most people make comments on blogs is to improve there link popularity therefore helping their serp rankings. I have no problem with people making comments for this reasons, the problem I have is that it can easily be duped and manipulated from people pimping that their blog is a do follow and then later down the road switch to no follow with one click of the switch. Which is very easy to do. I visit a ton of sites that claim to be do follow blogs and they are not.
Another reason is though google may give weight to do follow links now don’t think that won’t change down the road, google doesn’t like to keep the same algorithm and give advantages to websites that optimize for google and has to constantly change their algorythm to keep things on an even playing field somewhat, of course we all know you still can optimize your site for google but what methods you use now will most likely be changed down the road.
I try to find blogs that interest me and make comments on sites that appeal to me but not neccesarily because I think it will help in the serps more to build relationships with other fellow bloggers. Now my site right now is a do follow blog, with top commentater and comment luv so I really give link love to my readers that make quality comments. I do think do follow blogs are great but just beware that what some blogs claim to be are not always true.