I have a friend who plays premium and doesn't want to buy the shared bank for his characters but always complains about the price of mailing items. So, obviously he wants a shared bank, but feels that the price for it is unjustified. I believe he's spent about 60 dollars on the game, so it's not as if he isn't willing to contribute (and I'm not him, but pretending to be a friend of his).
I had an idea a while back for a 4-5 slot bank designed for players who really just want to swap items. It would cost 1/2 as much as the 20 slot bank and each of the other banks would stack on it, if a player later decided to enlarge their bank.
The advantage to Turbine would be that a player who gets the limited bank may end up desiring a larger one, perhaps spending more money than if they'd taken the 20 slot bank from the start.
The disadvantage, obviously, is that there might be a significant number of players who would just buy the small one, for exactly the reasons that my friend might, and never feel the need for a larger bank. Turbine may consider that some of these might have spent more for the big bank, if the little one never existed (as it currently does not).