It's nothing you can't do with pkhex and a computer, you could have multiple savegames and use their 31 boxes as a bank; having it in the 3DS just makes it more convenient.
This is a little offtopic, so you might stop reading:
Also, IMHO pokemon bank should be a one-time paid solution and offline, or at the very least poketransfer should. Paying to transfer genV to genVI...
And there's no point in the bank being cloud storage since you can't access it from any other 3DS, the only advantage is the unlikely outcome of losing your 3ds or accidentally formatting it...
I guess the true advantage will be when gen VII appears, since the transfer will probably be done as batch, but if gen VII pokemon aren't as gen VI they might just pull of a different app for genVII and make us pay again...