It's pretty simple. Deciding on what a "good post" is, is hugely subjective; like some people in this thread already said. As long as you think the person deserves it, then go for it. I just don't think you should give them hundreds in one go. Probably only give them a decent number of merits, depending on how useful their posts are. If you think a certain person actually deserves the rank up, probably spread the merits out on the person's posts.

Yes, you are to decide to whom to give the merit to. But if you give merits to people who don't deserve it but you just wanted to help them, then that beats the purpose of why we have the merit system in the first place; to potentially prevent spammers for joining campaigns, and or to prevent spammers in general.