I think it would help because people with as bad a memory as mine sometimes need help reminding them why they followed a certain person, for example.
Well, looking at the way posts are laid out up top and how comments are laid out, I think the best place to put it would be between the username and the post time stamp. That way, it's not confused with any of the other buttons or data that's on the screen.
I could see that, if only the person who flaired or tagged them could see it. Otherwise, you could have people tagging anyone that they don't agree with as whatever negative thing they wanted.
This seems like a decent idea. If mk and co. don't look into doing this, I might start keeping a notebook with names to keep track of who I enjoy seeing post and why.I think it would help because people with as bad a memory as mine sometimes need help reminding them why they followed a certain person, for example.