Interesting point, what if we changed the wording? from "The Following User Says Thank You to username For This Useful Post:" to "The Following Users Say Thank You to all For This Useful thread:"?
I personally feel grateful for all posters, I also don't feel the need to be thanked for every post since we have a rating system. There will always be that amazing deal that should get a full 5 stars & a note however that's not every post.
It's hard for me to get behind using "thank you" posts as a personal recording system, up post counts, or for attention since it doesn't have anything to do with being grateful to the poster? If the issue is honestly to say thank you, then the thank you button for the thread & rating systems handles it for both sides.