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.
Some might think that way, but if you've been to Reddit, there are both karma system and awards. Karma is basically the upvotes/ downvotes you get from others while awards are mainly paid gifts or customized Merits.
Therefore Reddit's awards are harder to earn than Bitcointalk's Merit but good contributions/ posts are often awarded with them. Just like Merit and likes.