Writing a good post worth a Thanks! costs more effort than thanking someone for a post. I assume you don't surf the forum to search for posts to Thanks!, but that you stumble upon them and respond by thanking.

There has to be a mechanism so that little johnny can't t(h)ank 1000 random posts to karma whore and screw the worthiness of the Thanks shown & search feature. I suggest, only let one thanking count to karma if there is an equal Thanks! given to the thanking user. So if you Thanks! and make good posts you eventually receive karma from both Thanks! as well as thanking others. This creates a healthy synergy between the 2 IMO.