Since Bitcoin-Qt is already one big mess, with like almost 10 dependency libs, why not to turn it into even a bigger one, right?
Payment protocol does not increase the lib dependencies in bitcoin-qt/bitcoind. Have you looked at the implementation? it's pretty small.
Point taken, however this makes the possible ramifications even worse.
Because the implementation is so efficient and bloat-free, the risk of it becoming "standard" way of doing transactions in the future is even bigger.
So as I already wrote, 15 years from now it may not be possible to buy something using Bitcoin without trusting some CA, which are controlled by government.
But well, if using PKI stays optional, then I think I can live with that. Still, I hope most people will not be using this broken (as of today) functionality.