I think you completely misunderstand how full frame works.
Hi Radiating. I appreciate the very thorough explanation!
I do not "completely misunderstand how full frame works" in that I do understand the relationship of DOF when it comes to just DOF...what is in focus with the same lens, crop vs. FF.
This is most apparent in video, and why FF and the 5D II changed the film world - the ability to blow out backgrounds and create that super thin DOF "look." I have also experienced focus pullers on set having problems with DOF that was too thin, and resulting footage that looked properly focus on the lcd screen but turned out to be soft when viewed at 1080p. Disaster. This is why I would always use the 7D over the 5D. It was more forgiving for focus if the puller was a bit out or the talent moved or swayed or slightly missed the mark, and at f2.8 still allowed shooting in fairly low light.
OTOH, now I'm more experienced with pulling focus myself and in selecting DP's, and plan (hopefully tomorrow) to buy a FF body instead of another crop. Having 2.8 and the option to go with shallow DOF compared to what is available at f4 would be nice. Suffice it to say that I would still prefer a 2.8 IS in a comparable range if it was available. If I want to buy a crop body or dedicated video rig later (like a c100/c300, BMCC or Scarlet), having the 2.8 would also be desirable to get the DOF down and keep the light transmission up. Naturally, IS for video is also a huge benefit in all ways. But I digress. This was a photography question, and I recognize that at some point, I may need to buy a 17-55 2.8 for the crop sensor dedicated rig I buy that will be s35. That lens cannot be used on the 6D.
The rest of the info about light and angles and stuff is all new to me, and I thank you for that. I'm going to read it one more time to let it sink in a bit better