I would call this "75% manual mode": you decide Av, Tv, and EC, the camera decides ISO. :-)
It would be useful in movie mode as well. For example, on 6D, my current options seem to be:
A) Full manual exposure: you decide Av, Tv, and ISO, the camera decides nothing.
B) Manual exposure with auto ISO: you decide Av, Tv, and set auto ISO, but since there is no EC setting to adjust (for auto ISO), the exposure is fixed at zero EC
C) Automatic exposure: you decide Av (assuming manual aperture lens), EC, and auto ISO is always active, but you have no control over Tv.
Tv priority movie mode unfortunately does not exist. If it did, it would be a workaround for me and other manual aperture lens users, as you could set Av on the lens, Tv and EC on the camera, and then activate auto ISO and let the camera take care of the exposure based on your chosen EC setting.
Canon could have just made the A, P, Av, Tv, and M modes work in movie mode just like they do when shooting still images (in terms of exposure related settings).