At 2512 x 1068 and using a 1000x CF, the 7D can still record continuously with sound but in my tests only at 17FPS. Is is useful? I believe so for certain applications or situations.
I remembered that back in the day our business shot events using 8mm at 16fps. The GoPro Hero3 can do 4K at 15fps. So at 17fps it must still be ok.
On the res chart flutes, this setting by far is the cleanest of the crop modes resolving past 1000 horizontal while 2048 x 930 is at past 800.
For normal human movement at a certain distances the motion is still ok, here's a short test. Watch it at full res.