With the possible exception of full resolution footage off of something like a RED One (never seen one, never seen the results from one), you can't pull decent stills from video. I have seen still captures from super-expensive HD Broadcast cameras (the $50,000 machines), and they're barely adequate for the newspaper - the place they sometimes pop up is in the sports pages - if nobody got the runner sliding into base with a 1D mk IIn, but the TV guys have a great shot, the papers will sometimes run a frame grab off the HD camera. You can tell even in a newspaper when they've done that (aside from the photo credit saying thanks to NESN, the image doesn't look as good as a true still). This is the extreme case - $50,000 video camera, very low quality print process hiding a lot of the flaws - and it still doesn't work!
The other example of still and moving pictures not being interchangeable is that Hollywood is VERY reluctant to print a single frame from a movie for advertising (they'll do it only if they have no decent choice among the stills, and only for a publicity photo, NEVER a poster) . I learned this from a still photographer who shoots on movie sets for a living! Even if they're shooting the movie in 70mm, they always have somebody with an M8 (quiet is VERY important) there shooting stills as well. I imagine that the quality difference there is because of the way movie camera shutters work or some similar factor, because 70mm SHOULD be equivalent to 645 stills if all else were equal - the presence of the still photographer shows that all else is not equal. I don't know if the RED camera will change this - I think its shutter is electronic, so if it is the shutter, the RED may be able to do it...
-Dan