Because a "controlled emergency ascent" is an emergency procedure. It's getting to the surface as fast as possible without blowing a lung. That's why every single agency teaches CESA as a "blow and go" operation. If someone has hold of you, is dumping your air, comunicating with you and controlling your ascent, it's no longer an emergency operation. It's a just a regular controlled ascent without a safety stop.
Yes a CESA is an emergency procedure.It is not about getting to the surface fast as possible.Its about getting to the surface in a safe controlled manner as to lessen any chances of lung expansion injuries. In a recreational dive situation ,I would not worry too much about DCI as you would be in limits and a DCI hit is something that can be treated. Drowning cannot be fixed,lung expansion injury can be very serious. "blow and go" option is so dangerous to that is it no longer considered a safe option for PADI or any agency I teach for.Who recommends "blow and go"???.Maybe 30 years ago , but not today. Also if someone has to have a "hold of you" and dump your air, and control your ascent, really doing everything for you because you are panicked or lost all control of your abilities, it most absolutely is a emergency operation for all involved.Why is not a alternate air source used if there is someone there to assist in holding you and dumping your air?? .. If there is someone there just perform a simple alternate air source ascent ,complete with a safety stop ,if the dive dictated it.That also is considered an emergency.. Anything else but a safe enjoyable dive, with no incidents, is considered unsafe and an emergency.