Originally posted by cmohr
I was thinking about this whole thing myself, as to the "why" bit. Why no support for the AA . and then it came to me, the K-3iii has the ability to charge the batteries, so if you had a set of normal AA batteries in there and connected the camera to a computer to download images, it would also start to try and charge the internal batteries of the camera, and trying to charge "non-rechargable" batteries would not be good.
Aha, that's a good reason.
I had a K-50 with the AA-option, and I think I did keep a few rechargeables in the bag for it, but TBH I never had a reason to use 'em. None of my cameras since worked with them anyway so nothing I ever saw as a negative.