At the other hand, from the paying side, when I get to a merchant's web page that gives me SSL authenticated bitcoin deposit address and the amount I ought to send - why in a world would it not be enough for me?
Why would I need an additional, payment request, signed with exactly the same certificate?
Good luck taking your screenshot of the "SSL authenticated bitcoin deposit address and the amount" to court when the merchant claims you didn't pay.
In other words, you don't really understand which problems the payment protocol is trying to solve.