Downvoting would be a little different. Since there are probably a lot of people that are higher ranks from before the merit system, it wouldn't be that great to give them that much power to shut someone down, so let's say that there would be a threshold where when you get enough of the downvotes you lose merit (maybe something like 0.5) but if you get downvoted by a legendary or some other higher rank, you'd lose merit when you get downvoted regardless of the amount of downvotes you have or do not have.
I don't think that introducing downvoting that can lead to loosing merit would be a good idea. Rest assured, if something can be abused here on bitcointalk, it will be. For example, let's say that you caught group of high ranking scammers/cheaters, they can easily retaliate and make you loose the merit. Would that be fair?
So if someone posts something really useless or copy/pasta reply, he can get downvoted enough not to lose merit but to think about what's he posting next time. Like a small slap on the wrist.
You can achieve the same if you report useless posts to mods. Chances are spammer will think twice before posting crap when he sees his useless posts constantly getting deleted. Every once in a while we get someone opening topic about his posts mysteriously disappearing, so they do notice that.