Who the heck would want a FF lens that only goes to f/5.6 at 105mm?Not me, but people that want a light, cheap lens for landscape, travel, & studio use where shallow DOF is never used. 6D shooters would likely be the target for this lens.
Your logic makes about as much sense as saying who would want a crop sensor, or would want a camera any less tough than the 1D X.
There's this little thing called market segmentation. It's how big companies make money. Reference Canon's profits on the Rebel line vs. the 1D line.
Well, I am a 6d shooter, and I can assure you that I would never have invested the money to go full frame if I had any intention whatsoever of shooting a FF equivalent of the Rebel's cheapo kit lens.
If they want to offer a less-expensive kit lens with a FF camera, they should simply offer a kit that includes a 50mm f/1.4. That could be done in a white box for just a couple hundreds bucks over body-only price and might be very popular.