I think it can be a convo capper because sometimes people use "thank you" out of obligation more than really expressing their gratitude. I'm sure this is not the case for everyone, but is for some.
While in college studying PR, a guest speaker at an event once told me that people want to feel appreciated, so to build relationships, let them know specifics about what you liked in an article, blog, or comment they've written.
Someone that does a great job of this is Lisa Petrilli (
www.lisapetrilli.com
). Her responses to comments on her blog are always very sincere and always make you feel part of the conversation.
I don't ever think saying "thank you" is a bad thing (do I sound like my mother yet??), but maybe adding something more will help others feel comfortable in replying.