Lots of defencemen can handle the PK. Heck, on the PK, Benn is almost as good as Weber. At 5v5 or PP Benn isn't close... but just PK work, he's close. Alzner is also going to be a good PKer, and so is Schlemko.
Not as many can do what Shea Weber does on the PP.
PP are also much easier, much, much easier minutes on a defenceman's body than PK minutes are... (no blocking shots, no chasing the puck, Weber isn't battling in front of the net on the PP).
Basically if you want to cut a defenceman's minutes, PP is never the area that should be cut.