Hello, I would like to come back on this idea.
Personally, I think this is a brilliant idea! I recently purchased a WD MyCloud and I must say that this would make the product more convient for me.
I have noticed that one of the philosophies behind the MyCloud is to provide a simple way of storing your data. There are more people who are interested in multiple Time Machine backups and the current expierence is not easy.
Setting a limit for shares is like making a partion but thanks to the myCloud dashboard, the UI is easy to use and powerfull. The advantage as referred in the first link is a great example with the kids, and even if this is not the case, the possibility of limiting the size of shares gives a feeling of being in control.
I hope that you will take a second look into this idea, and I would love to hear what you have to say,