The only drawback of having both the merit and the like button is that most people will like the post without given the merit (as merits are rare and precious).
I personally don't like this idea. Again it can be treated in number of different ways like you press the like (shown publicly) and other people may give them merits thinking that the person who liked the post didn't have smerit to spend.
Your first submission about the purported draw back. I don't see it as a draw back, if you continue giving likes and bank merit, where will you use the merit later? Still in the forum.
About the different way you suggested to treat it, you cannot achieve it without first adopting the like system alongside the merit system.