Oh, and RE: privacy:
Mixing all of the 'coins' for all of the accounts together is, I think, an overall plus for privacy. True, you might receive coins from somebody and be able to figure out "Hey, they sent me some of exactly the same coins I deposited in my BigBitBank account yesterday! They must have an account at BigBitBank, too!"
But mixing all the accounts' coins together makes it much more difficult to figure out where any user's coins came from, or, if you know where they came from, where the user is spending their bitcoins. Unless you can subpeona BigBitBank's records, of course...