Don Haines said:agreed!
Now a FF mirrorless..... that's a different story!
I guess you guys are right, there's no reason I can think of NOT to use an auto-detect EFS mount on a full frame camera. I cannot think of any downside. if you had a 50-ish megapixel FF camera that become a 30-ish megapixel APSC, that would be neat.
On the other hand, I also can't really think of any EFS lenses I'd want to mount onto a full frame camera, except maybe 17-135 (which would crop 1.6x that), as it is a very nice lens for video, with its very quiet and fast AF motor, that also allows for electronic W/T control.