Unless you can shut down the SRB, aborting a CEV stack could be much more difficult. The abort rocket would have to either pull the CEV away from a booster that's still pushing very hard (meaning the abort motor would have to be very powerful to overcome the acceleration of the SRB) or pull it off to the side with enough lateral acceleration to get clear of the SRB exhaust plume, or more likely a combination of both. Any of these options means for a very rough abort, more powerful abort motors, and heavier structure to handle the loads.

Aborting from a still thrusting SRB is analogous to trying to eject from a fighter that's falling rapidly to the ground and is very close to impact. When the sink rates are high, the ejection seat may not be able to get the pilot clear of the plane and get his chute open before he hits the ground. Unless the pilot can reduce the sink rate, he's going to die. The Thunderbird crash last year was a good example - the pilot knew he couldn't complete the maneuver and was sinking too fast for a safe eject, so he stuck with the plane long enough to reduce the sink rate. He got out about a second before impact.