There's nothing wrong with doing it,
a bit OT but i wouldn't say nothing because there are a couple of things wrong with using a bitcoin address/key instead of a PGP key.
yes, I agree with your reasons completely.
However, I think that the very fact that Bitcoin introduces people to the concept of signing messages with crypto keys is actually important. To put it another way: signing messages from Bitcoin addresses works, but it's far from ideal, PGP is far better for signing/encrypting (etc...) of messages for regular people. Proving who really owns the keys is more of a problem, but Bitcoin is even worse in that respect