I guess if you were caught in very poor light, without a tripod, then the highest levels might allow you capture something that you otherwise might not. But that said on the K-1, the highest I would normally consider using is 51200 (and then take a few shots and hope for one without blur).
With some PP NR (in RawTherapee) the best colour image quality I got with the K-1 and ISO 204800 was:
Obviously the results look better if you convert to black and white (where some noise can look interesting anyway).
The funny thing is it was a very dull day but I was shooting at 1/8000 and f8!
ISO 800K on the K-1 II looks a lot worse again (no idea why they bothered!)