Tuheeden, you do realize that paper wallets cannot be passed around like cash, don't you? The private key is printed onto the note, there's nothing preventing someone from making a copy of it and then adding it to his/her wallet after the note has been passed to another person. And if only the public key (the Bitcoin address) is shown and the private key is hidden or covered by a flap, there's no way for anyone to check that the owner actually knows the private key associated to that address.

Essentially it's only valid for a single transaction, the recipient should immediately import the private key into his wallet and swipe the funds to a different address.