Well I don't believe the diamond's market should be banned for bankers... Since they will no be able to send them to other accounts. As we said bankers will be not allowed to do transfers... It will only to affect prices/market, one of the reasons to be someone banker.
Why someone to become a banker? It's easy! Free gold! You give loans and you take back your money plus something, also when people deposit gold to you then you are able to buy better/more stuff of everything and also allows you to give better quests to other players (if applied) and gaining even more from their quests.
Here I would like to add that among the regular quests (crafting, gathering, dungeon fighting), we should add a head hunting quest!
This type of quest will be acceptable from many players, and once one of them find the wanted player and win a loot battle he can complete the quest (the quest becomes inactive for the rest). But the loot going to the quest giver, and quest taker can take only the reward! Now... The bankers will unable to deposit somewhere their gold (somewhere safe at least), they can give it only to other bankers, thus making perfect targets for head hunting. But a "clever" and "strong" banker will be able to win much more gold in such way, than losing it...
And another "rule" I would like to add about the bankers: The max loan should be the min deposit. So we will never see only loaners.