In the beginning there was Minecraft, but it wasn't long after we started talking about Scrolls (and later Cobalt) that we realized we wanted players to be able to use the same credentials for all games, and this is where we currently are headed.
The idea is that you use this Mojang account (where the username would be your e-mail address) to sign in to Minecraft, to Scrolls and to Cobalt. After that you would proceed to choosing which profile you'd like to play as—for Minecraft this would correspond to today's accounts (premium status, display name, skin etc).
The reason for this change is first and foremost to keep things simple for players who wish to play our other games, or to have several Minecraft accounts under the same roof. And oh, we won't just shove over the almost 18 million (!) current Minecraft accounts to the new system but you'll have to opt in, at least for this point in time.
We are planning on rolling out the service with Cobalt some time this month and then fully integrate it with Minecraft early next year.
If you have any questions, comments or feedback I'd love to hear from you in this reddit thread.