Artemesia, you're failing to factor in a question: How did the banker get all the gold to begin with? It has to come from somewhere, be it wealth taxes, nobles turning it over before they leave on a campaign, etc.
And yes, there could be the "action banker" scenario. But when bankers are frequently targeted by assassins or enemy patrols (their gold possibly being taken when thrown into an enemy dungeon), how willing will these far flung armies be to see that wealth suddenly vanish?
It simply gives us all another tactical option. Sure, it can extend a campaign, but it won't make it indefinite.
If this was added though, I'd suggest giving infiltrators an option to rob bankers, similar to the current assassination mechanic. Perhaps give the Banker an option for how heavily he'd resist, much like the current dueling options, and if he fights and loses, he may not only be injured, but lose X amount of gold.
There are many scenarios I can imagine if this happens, mind you. Could you see a banker waiting in an allied city for the army to report in for a quick gold trade, only to be stabbed and robbed by an unknown assailant? What if it happens repeatedly? How long would that nation remain an ally before suspicions tear the alliance apart?
