but they deleted your stuff after 90 days of inactivity if I remember correctly.
My personal guess its not about storage but as you pointed out - processing and bandwith
But You dont need to process chars You dont use, You have other save file for Wizard and other Witch Doctor for example. And syncing items is just downloading them to memory at first and then checking sum from time to time to avoid modifications on client side.
BTW i've started Diablo 3 and checked how many data it transferred to me.
For recall to town and this many items
http://i.imgur.com/kPVsP.jpg
i got 122kb of data ...
http://i.imgur.com/5tjjn.jpg
And thats with login into the game, moving, syncing etc.
It's more about managing the number of calls to a database than it is about storage size. They are effectively lowering the number of accesses to their database by 40% with this change.
DB administration can be tricky and I am sure you would much rather have three tabs of storage than be shown a buffering symbol for 3 seconds as your storage items slowly populate into the GUI.
Its not like they couldnt stream it in background when You enter town? Those all items land in Your local memory after all and they dont search db for stash when You are in dungeons.
They could make also non shared stash like they wanted at the beginning and store some items in char save file.