Using PayPal gives you my email address. I understand I'm probably a minority in this, but I don't tend to give that out anywhere on the web and I don't like that I can't hide it from the recipient on PayPal. No I don't care what you're "allowed" to do with it, it only takes one bad actor

That said, I don't think there are any sites I really frequent any more [though HN is one I scan often], I've not noticed PP tip jars on the small number of sites I browse.