I think this would be a nice idea. Thanks isn't the same as merit or activity, it's more of a personal message to the poster.
It isn't worth putting much effort into adding it to the forum, but if it is easy, then I think it would be a useful addition.
I'm with Jet Cash, this is a good idea for me since giving thanks will also reduce people who shit post like " Thankyou for.." and also giving genuine thanks will also make people happy and appreciated and they will continue to strive hard.
The OP stated that "thanks" button will not be same as merit since it will not be a rank requirement i think this would be just work as Token of Appreciation.
+1 to this suggestion if this is not that hard to implement to the forum.