Originally posted by Dartmoor Dave Canon, Nikon, Sony, Leica (et al) don't include pop-up flashes on their "pro-level" cameras.
Depending on your definition of 'pro', most Nikon current FF's - the D600/610, D750, D800/E, D810 - have built-in flashes. The only current FF models that don't are the D4s and Df.
IMHO, if it's good enough for the D750/D810, a built-in flash will be good enough for a Pentax FF.
It's true that every Canon FF from the 6D upwards lacks built in flash. And it's true that the only current Sony FF with a built-in flash are the RX-1 models. But I'd prefer Pentax follow Nikon's path on the matter.
Built-in flash is useful not only for convenience and fill, but also as a handy wireless flash controller, as others have noted.