Years ago, I had a Nikon FM-3a with the Nikon battery winder/grip. That was a film camera; the autowinder grip made it easy to take pictures fast... which made it valuable even though it added a lot of weight.
Digital is different, though, If I can get 300 pictures off one regular battery, and then carry a backup in my shirt pocket, why on earth would I need to put up with all that extra weight?
I guess it's for the video people, so they could run out the full 25-minute recording time, swap in a few more AA batteries, and shoot that scene over again. If the Pentax HD format is close to or the same as Hollywood's 4K video standard, the K-3 would make a GREAT field camera for nasty, watery, muddy on-location filming. See
http://en.wikipedia.org/wiki/4K_resolution