Well, the reason that M0 is the basis of magic isn't some great mystery. It's quite simple, really. It creates a foundation on which spells can be built. If a magic zone is completely filled with magic, then in order to cast a spell, you need to clear out an area/object of its old magic so that you can create the effect you want.
As I said in the discussion/prediction thread, though, I'm pretty sure the secret is that Black Plates can use magic outside of the school. The school exists to find exceptional people who can master magic and qualify for using it outside of magic zones, for some mysterious purpose.
The principal told Kuzumi that Black Plates would be useless for normal mages because they'd only be able to store their own spells, which naturally they can already cast. But since normal mages can qualify for Black Plates, logically there must be a situation in which it would be useful - and that can only be when they are otherwise unable to use magic. In other words, outside magic zones.
Now, what the mysterious purpose behind this is, that I don't know. I wouldn't be surprised if there were some sort of tournament between different schools or something though, since this is a Shounen manga after all.
- nice twist and suprise - nice magic/skill chapter
