This is a centralized forum, and even though it's about something decentralized, it shouldn't be. Imagine how much worse the spam situation would be with no moderation at all.
What's stopping me from making hundreds of alt accounts to easily gain merits? Your idea gives power to the account farmers again, which is a horrible idea and restarts something that shouldn't exist. I think we should wait a bit more time before jumping to conclusions with the merit system, and if you consistently post quality, missing out on some of those posts shouldn't be an issue. Only the people that are here solely for signature campaigns and spam dislike the merit system.
Good point, perhaps any users who have member status or above should be able to upvote, and those who are newbies and jr member shouldn't. Anyway, I think it would be far more profitable for users to spend time making quality posts and gaining merit that way instead of making loads of alt accounts for only a few merits or less. Bitcointalk has a huge user base to dilute alts.