The thing is, some Member rank and even up to Sr Member rank also post useless reply for signature bounty sake.
Merit will definitely help for accounts level Jr Member and below but those with Member rank and above, I don't see Merit as an effective solution to get rid of useless post from existing higher ranked member.
It might not be and that's why there are other options being looked in to, such as the trusted members of the forum being able to tag accounts as spammers, that's just one of many suggestions you'll find looking through the meta section. There's no solution that's going to work for everything but merit should at least be a start.