When I think of merit being used as a thank you, I think more along the line of you had a problem and then someone helped you with a solution. Leaving their post a merit would be a nice thank you. Just leaving merit because someone left it for you is actually bad, because it could lead to users thinking they are involved in merit circles to inflate each other’s scores.
I do agree with you... Been reading about the merit system since i joined, and it's only fair that merits should be for posts that goes to help people instead of making it a return of favour.