Post are not suppose to be merit more than 2-5 depends on how helpful and relevant the post.
actually there supposed to be no rules for how much merit you give away, it is your spendable merit and you can use it however you like.
People make good post that deserves to be merited but once you come across such post that normally you think it need to be merit but occasionally the post already has a good merit I think no need to add more to it
let me ask you this: when you go on reddit for example and see a post that has a bunch of upvotes, won't you upvote it just because it already has some upvote or will you do it despite that?
that the same with merit. when 10, 20 different users give merit to a post they are showing that they have found the post useful and the amount of merit they give it shows how much they think it was useful.