I said this once on another post, I'm going to say it again. Can you sell parts of a game that were included in an original purchase, made by a specific content creator, and sell them for your own profit? No. It's in their EULA. It's very clear, and it's very common sense that you cannot sell someone else's work for your profit. This is only a big issue now, because it has become people's jobs to make servers, and charge money for items that are already included in a game. It's selling mojang's property that they created and have rights to. Arguing against that is pointless. If you like a server, they have no issue with you donating. Only with the servers selling items for actual money. So no, it won't ruin good servers. Because if people actually like them, they will make sure to donate and keep them alive. Even without benefits.
I think if we get this "cool looks" like prefixes, nameglow, talk in color (etc.) Mineverse will do alright.