I use "follow-up" as a positive tool. When I see someone leave a negative follow-up to a negative feedback, they start to come across as being obsessive. Actually, when a seller doesn't leave negative feedback or negative response to feedback, I tend to "side" with the seller. Sometimes not saying anything is better than saying negative. And, I'm not sure actual feedback text is as important as the counts.
As a positive tool - if a seller has provided special service, and especially if they specialize in the area I'm interested in, I'll leave a positive follow-up. No one but the seller will probably see it, but they usually remember it, and since I tend to buy from pretty much the same sellers, it's worked for me.
As on this forum, people tend to be more passionate and frequent with negative than positive.