Peter Haas over at RedSharkNews pens, what he believes to be the definitive answer regarding when you should shoot RAW. I'm not positive this is the definitive answer, but he does have some good points that are worth considering if you want to hack your 5DIII, purchase a Blackmagic camera, or scoop up a new D16:
Shooting raw video seems to be a very loud and persistent buzz from the filmmaking and shooter forums these days, especially so since Magic Lantern announced they had successfully hacked Canon DSLRs to shoot 24 frames per second (fps) in the raw image format. While there is a lot of information out there about technical side (what memory cards to use, how to install the firmware, etc.) there is another whole side to adopting raw image formats because this migration is going to completely change how you shoot.