It's an extra point of attack on those coins if someone else also holds a copy, and you have to trust bitbills.com just a little bit more.
Some would prefer to think that they hold the ONLY copy with no backup.
Some would prefer to think that they hold the ONLY copy with no backup.
So whenever you meet (or communicate with) somebody who owns bitbills check your bills' public address against their bills' public address. If there is significant counterfeiting going on, eventually you'll find a match. Try to redeem both and you'll quickly find out which is real and which is counterfeit (or that both are counterfeit).
I was going to suggest creating a public Google Documents doc where people could enter their bitbill public keys, but griefers could just look at the block chain and pretend that they were holding bitbills that they don't actually own.