I don't like the fact that posting is in some way punished.
When you put it that way, I kind of like this idea even more now.

It will make shitposters think twice before pushing "Post" under some text they know is pointless garbage. Which, let's face it, is at least half the posts on the forum on any given day.
But I don't like that boards with low merit circulation for whatever reasons will be overly affected negatively, maybe shutting down lively discussions because of over-amplified "carry".
Sure, some boards are definitely under-represented in terms of merit. However, most forum members don't post solely on one board. And again, we're talking 1 earned merit for every 10 posts. This shouldn't be too hard for anyone to do. If it already is right now, someone can't earn 1 merit in 10 posts should seriously question what it is they are actually doing here.
Actually I can imagine who will it affect the most: bounty hunters. There are hundreds of accounts with less than 5 merits and thousands of posts. They will quickly go from Jr. Member to Newbie. Maybe some boards could be carry-exempt, like Bounties, Dev & Tech Discussion, and Serious Discussion.
Spammers already can't rank up so easy because they'll hardly get merit for their shit-posts.
Ah but they often have "friends" that give them merits for their shitposts, enough to rank up, anyway.