If you aren't obsessed with controlling the trust network and you don't care about ranking up, there's very little benefit other than a dopamine hit I guess.  If you earn them you can also send them to others, so I'll shoot you 2 merits so you can try giving 1 away. 

I guess the main benefit is that signature campaigns pay more money the higher you are ranked, so if you make good posts and earn a high rank, you can get paid more for posting.