Losing the manufacturer-native RAW file feels akin to throwing away negatives just because you have some prints (okay, it's not that bad).
It seems logical that camera manufacturers are in a position know their cameras, their lenses, their systems, best. Though DNG is extensible, it's not controlled by camera manufacturers, and innovations they implement may conceivably not be easily reflected in DNG. They could, on the other hand, more easily extend their own RAW format(s). On the other hand, if they were constrained by DNG, it could conceivably stifle innovation. Is DNG always good enough for everything? Or is DNG just better for Adobe?
What if DNG becomes a standard and needs to be extended to support a new feature Canon rolls out, but Nikon has more sway with the standards board? Or maybe Canon wants to keep it proprietary? DNG breaks down and everyone's using their own formats again, and DNG. You'd have been better off just using a camera-native RAW format all along.
As for workflow, i'd support conversion to DNG if it makes software better, but if my Nikon saves its best image as NEF, you can be sure i'm saving that NEF file, even if i convert to DNG. If my next Nikon spits out DNG files, i will only opt for that if i can't get a NEF from it.
If your Pentax does both, i'd stick with the PEF file. If you use DNG in your workflow and have card space to burn and you'd rather have the camera do the DNG conversion in-camera, take advantage of that and save both. But throw away the PEF file? No way. What if your next Pentax doesn't save DNG? What if DNG eventually goes away? Will you some day kick yourself for not having that PEF?