But you can't prove that the owner of the PGP key is the same as the owner of the bitcoin address, because the bitcoin address is the public key of the keypair. What is to say that you try to spoof someone into thinking that you own an address belonging to someone else by taking one of their addresses and signing it? I don't know why someone would want to do such a thing, but it could form the basis of a 'man in the middle' attack.
True, using a system outside of Bitcoin itself, opens you up for loads of theoretical and presumably also a couple of real life attacks. Using PGP however does allow establishing trust in a certain way: a bitcoin address signed by a PGP key held by a friend, or a friend of a friend, should indicate that there's a kind of trustworthiness associated with that bitcoin address. After all, the one who holds the PGP private key has announced the authenticity or ownership of that particular bitcoin address.
Now, it could be that the private PGP key is compromised. In that case the original owner should have revoked the PGP key, thereby revoking the associated trust. Also, if an attacker gains access to your private PGP key, there would usually be very little between him and your bitcoin wallet as well, so a compromised PGP key would most likely mean your bitcoin wallet is compromised as well.