Ok the titles a little misleading as the 645Z is a Medium Format Camera and Full Frame refers to 35 mm Film equivalent, However the argument for 35 mm Film equivalency is often stated as having greater depth of field, wider angle of view, higher ISO sensitivity but the way technology has increased it could be argued that ISO is no longer a factor. This is true of the 645Z and as the sensor is larger still (35mm = 0.62 of the 645Z)
Pentax actually make a superior camera to the oppositions 35 mm equivalents (barring all the functional items in memory which don't really contribute to picture quality).
Consider this.
A 55 mm Lens on35 mm equivalent camera is = to 35 mm Lens on a Pentax K3(Pentax K3 has a crop value if 1.4) With this you can see a 12 mm lens on a 35mm equivalent as apposed to a 12 mm lens on a Pentax K3 is going to have a wider angle of view (great for landscapes), 12 mm on 35 mm equivalent = 16.8 on a Pentax K3 At these wide angles you run into Lens manufacturing issues in trying to get less than 12 mm and you would need a 8.5 mm lens for a Pentax K3 to be equivalent. But what about the other end the telephoto end, its a greater advantage to have a Pentax K3 i.e. A 300 mm lens for the Pentax K3 = 420mm for a 35 mm equivalent. 420 mm Lens's are much bigger and weightier than 300 mm lenses.
So it depends on what you want to do with your photography, if you take a lot of Landscapes and predominantly take landscapes buy a 35mm equivalent or better still buy a PENTAX 645Z with a 22 mm lens its better, and still has a wide angle view of a the 22mm lens for the Pentax 645Z is = to 15 mm 35 mm equivalent. If your into sports or birds stick with your existing Pentax DSLR or get a PENTAX K3. Anything in between and your existing Pentax will still take great photo's.
I like quite a few people would not be able to tell the difference between a picture taken with a 35 mm equivalent Camera or a Pentax K3 (or similar). I recently had a Pano from an iPhone printed on a A1 sheet, the quality is excellent and if you seen the picture most would say how may shots did I stich together from my Pentax.
I can only guess BUT is this why Pentax wont make a Full Frame Camera (shudders at using that term) Pentax have a better camera at a physical size not much larger than top 35 mm equivalent in the 645Z Medium Format Camera.