True, it may be better to use a signed message, but thats the reason why we're quoting messages of the previous user - So that even if someone were to have access to your account and edits the post, it would easily be able to be seen that it was changed based on the quoted post from another user. Also, to read PGP key messages, they'd need your private key, which in that case you would be seriously compromised and prob have larger issues to worry about than just bitcointalk.