Nobody's saying that you shouldn't take simple steps that can inconvenience an attacker, and the concerns you cite about convenience, not getting unwanted email at the address, etc. are all valid. But you explicitly mentioned "giving out half the info required to log in", which is a security argument, and not a good one. It wouldn't be a bad thing for PayPal to let you decide what email the user sees, but switching to secret userids because they somehow constitute an additional security factor is misguided.