Suppose someone puts a bogus QR code under the sticker of a printcoins.com note. Is there any way to guard against this besides breathing down the neck of whoever gave it to you?
If you are going to extend trust to the issuer, then you might as well use media that is more durable and harder to counterfeit (e.g., Casascius physical bitcoins). Paper wallets work great because they are cheap enough to produce that you don't lose much when destroying them after one use. If I scan the QR code upon receipt (or when I get back home, etc,) then there is much less risk than circulating a piece of paper that potentially could be counterfeit without anyone really knowing whether it is or not.
