This point is interesting:
"Can I use Pokémon Bank as soon as it finishes downloading, or must I reach a certain point in my Pokémon X or Pokémon Y game first?
If you download Pokémon Bank to your system in the Nintendo 3DS family, you will be able to use it at any point after obtaining your Pokédex in Pokémon X and Pokémon Y. You cannot use it if you have Pokémon X or Pokémon Y but have not begun the game."
So I can start a game get the starter and pokedex, and put that pokemon in the pokebank. Then start a new game get a new starter and put that it the bank. Start another new game, get the last starter, get the other two out of the bank and have all three starters?
I highly, highly doubt it. Since the X and Y pokemon will have the various Trainer ID numbers, and every new game you start will have a unique number, you most likely won't be able to withdraw pokemon that aren't yours. The only exception being, predecessor pokemon from Gen V. And I presume they've also altered the numbering system to make them unique to X and Y, so it'll be easier for the system to tell the difference, since, as we know, there will be some older gen pokemon in Kalos.
And that's just the normal banking system. I don't think the Transporter will work as such, since the Regional pokedex won't have ALL the pokemon in it, and thus, wouldn't be able to recognize any pokemon you try and transport. That part of it will be like the Transfer Center most likely, and unlock when you receive the National pokedex.
Of course you'll still be able to store them. I'll only be using the transporter, as I don't actually see the point of the bank system
Up until now, with how many PC boxes there are in game, you can have one of EVERY SINGLE POKEMON, and still have room. And twice now, I think, they've increased the number of Boxes, since the total number of pokemon exceeded the maximum space. With the new additions, they will be exceeding 720, and they'll have to update the number of Boxes, since there will be plenty of people who don't want to use the Bank. If they go up to just 30, that's 900 spots, more than enough as usual.
Edit: Something else I remembered. A more pertinent reason for why you cant do this
The Bank and Transfer system isnt coming out til the 27th of December