You still have your wallet on your PC where you do your payments, speculations, etc. So this is more like an ultra-safe wallet backup than a cloud wallet or something similar.
So the advantage to me is that I do not have to worry if my computer fails, gets stolen (assuming I encrypted my wallet).
How often could I backup? I think the ease of doing the backup is going to define your market.
I would still have the fear of someone hacking my machine and just transferring the coins away though. I could, however, take the orriginal option and store the coins in a separate wallet if I wanted.
How many wallets can I store?
if someone would manage to find out that there are Bitcoins stored in the bunker (very unlikely) AND if someone would pass all the security barriers (like video cameras, security guards, multiple steel doors that are locked, etc., which is extremely unlikely), he couldn't do anything with the files.
I would be more concern with an attack from within. You can make it very hard to get into a place, however, if there are employees inside then there are already people inside that can do some damage of theft.