I don't know how I feel about the Minecraft Coins concept, but otherwise this seems like a reasonable idea. Since it doesn't impact free add-ons, and it will (ideally) make it possible for professionals to devote time to creating content, it seems like a good idea to me.
As for the Coins, I'm reminded of X360 Moon Points, which everyone hated. However I do get that, given the small scale of these items, that vendors don't want to set the price per-region. I feel like having fixed pricing tiers (at least below $10 or so) would be a better way to go. Coins and other virtual currency are too ambiguous; it's too easy to forget the real-world value of that currency and spend like a drunken sailor.