You know what? I think I agree with The Rusty Creeper, but I think it could mess servers up, too. I voted "Yes.", but I think they should add another feature called the "Multiplayer Server" accout . It will be like this:
Buy MInecraft (or get a gift code, cracked version [that's basically Demo with creative and save], whatever, etc.)
Make Minecraft Username. You can change this.
BUT, to go on ANY online multiplayer server, you must create another account that you CANNOT CHANGE.
This sounds pretty fair. This is ONLY A SUGGESION. If Notch decides to do this, please give me credit (suggested/proposed by "CreatorCMS" or "Creator".
P.S. mind you, you can change your Username in MinecraftSP as much as you like, and still go on "cracked demo-version)" servers. You can download that here.