I literally just had the same idea, and I searched to see if anyone else did... I like your ideas, but in my opinion, you left out some things that may make this sound better.
First of all, MC-Cloud would be a storage system, NOT a server for people to play on. Instead, it would only add/update a copy of your world that you could access anywhere. Your uploaded worlds would not update in real time, you would have to save it in order for the newest version of your world to be on the cloud.
When accessing your world, you would go to a special tab and select your world. The file would be downloaded replacing your current copy of it, then opened locally. With this method, sharing worlds would basically stay the same. However, there would be a button next to your world in the menu letting you send it to your friend's account. The following options would be given: have both of your copies updated when either party makes changes, only the receiver's copy is updated when the sender makes changes, or neither. If the latter is selected, the files would remain totally separate as you had mentioned.
You touched on some of the advantages of having this, but I am going to elaborate. MC-Cloud would let you open the world on any computer. Obviously, this is already possible. Just send the save to yourself, and open it on the preferred computer. However, you need to keep doing this in order for you to constantly have the same copies. MC-Cloud would let you easily access and update your local worlds.
For the record, this would NOT be realms. Like I said above, it definitely would not be a server. It would be a way to keep you and your friends updated on the changes you make to your worlds without having to resend it. And if anyone had another method of doing this, it really doesn't make a difference. This idea would essentially just give sharing an interface within the Minecraft game with other added perks.
As for cost, I feel like Mojang could afford to let people save at least one world to this cloud for free. After that, they could charge however much for any additional worlds saved.
Thanks WestSideSammy for posting your ideas, tell me if you like or dislike my additions. Remember, the key words are "constantly updated." It is a major part of our world today, and it's time it came to Minecraft.