You can have every Pokemon, every form AND have a lot of boxes spare to do lots of breeding.
This is one thing I look forward to doing assuming I'll use the bank beyond the trial (laziness is a thing, after all). I've never done a living dex before because of space and preferring to organise by type, but if I can push everything out of the way onto the bank that makes things so much easier.
Official Oblivion Keyblade of the KH3 Board 3ds FC: 1934-0765-5595
You can trade over your last XY pokemon when you decide to start a new game without having to have a friend with a 3DS and XY copy or having to go buy them for yourself. $5 a year better than $240 one time. It's the lone wolf option.