I always find this argument really funny. This is the one reason why so many newbies argue that it is really hard to earn a merit. They will post a decent and high-quality post and you will point out that they do not deserve it because of their other posts.
I am guilty of that as well. The problem comes from rewarding posters in the Bitcoin Talk underworld. Merits generate sMerits, and I try to keep them away from their potential abuse in the spamming underworld. Also, keeping merits away from active spammers may encourage them to leave, as you have pointed out, and I believe this is why the merit system was introduced.