A quality post is still one irrespective of the members previous post history, that's why merits were introduced, to encourage quality. This is just a suggestion.
I don't want to clog up this thread with a discussion about this, but when Theymos tweaked the system in September, when a lot of Jr. Members were busted back down to Newbie rank, there were a lot of them trying to make the best posts of their lives in Meta, hoping to get that all-important 1 merit. And usually the posts weren't all that good anyway; they were just longer.
The merit system was designed to suppress the shitposters from ranking up. I really don't want to merit the one post that's decent from a user whose post history is 99.9% garbage. I might do it if that one post was good enough, but it'd be a tough decision--because I don't like to see shitposters getting merits.