It would be great, if it would cut down on the number of ancient threads being resurrected years later, just to say thanks! I've never understood the sense of doing that, since you're not adding anything of value to the thread. Sadly, I think that even with a "thank you" button, they'd still resurrect threads to post "thank you" or "this really worked", etc.
I wish this button were available also. It would hopefully cut down on the number of +1 type posts plus possibly make it easier to find the "right" answer in a thread.