A lot of fine photos were taken with 6mp DSLRs and other digicams. I don't think the point with grabs from 4k video is that it's as good as the best IQ stills, but that's it's finally very very good - good enough to hold up as still photos. I think that if that way of capturing stills suits you, the quality is finally good enough to make it work. Not the best IQ certainly, but pretty good. Hell, it was just a couple of years ago I was happily shooting with the Fuji X10, a 12mp EXR sensor that got its best results pairing pixels to give you a 6mp image. And I still have images from that camera I'm real happy with. 9mp isn't bad and for a video grab it's basically amazing...
The shooting process doesn't sound like fun to me, but I'm sure it will suit a lot of people really well.... Particularly for shooting sports and stuff - just make a video and pull out stills of the key moments...
-Ray