Real banks give interest on deposits, to encourage you to give them your money so they can lend that money to somebody else at a higher rate. Who are guild banks going to lend money to?
BRE had investments, clearly wow should too. (/sarcasm)
I'd rather just address the costs of ownership for raid characters than try to break the server economy. They're addressing consumables to some extent (although not as far as I'd personally like) that leaves the repair bill issue. Perhaps have durability loss for dying in a raid waived. (This is what guild banks basicly do) In at least one analysis, the difference in total durability lost to intentional wipe between a raid a 5 man and a duo is silly.
Look at the total durability lost to people intentionally killing themselves (so ignore the first few people that died to the encounter, and total up the people who went swimming in the lava to get dead and started on their run back faster.)
The duo probably never has an intentional wipe - they're few enough that they just normally. Zero durability intentionally lost to save zero minutes.
The 5 man might call a wipe at 3 people dead, telling the other two to just die for a cost of 20% durability intentionally lost to save x minutes.
The 25 man might call the same wipe at 3 people dead, telling the other twenty-two to just die for the cost of 220% durability intentionally lost to save the same x minutes.
Another way to think about it is in total deaths per hour. I doubt any character in wow dies more per hour than the avg for an active raid during learning.
Maybe the first arbitrary amount of people to die in a raid/party should take the full durability beating, and everyone else misses it, or maybe it should just be waved entirely, but the disparity between costs of intentional wipes is pretty silly, especially when the "total number dead to require a wipe" is similiar.
Something like that would go a long way to eliminating the _need_ for a guild bank's "repair from guild funds" and would incidentally make raiding less burdensome, again shifting the ratio of time spent having fun higher compared to time spent mindlessly abusing elementals. Raiding is time intensive, and by it's cooperative nature, not something you can really just do 24/7/365. It doesn't need extra timesink handicaps, the organization is more than enough timesink.