That presumes the FA's are physically able to oversee the evacuation. Look at the recent Singapore Airlines incident, several FA's were among the injured. What if there is damage to the aircraft that required an emergency evacuation upon landing, and multiple FA's are injured? Too many people ignore the safety briefings now, so you'd be left with not only the disabled needing instruction and assistance, but the pax in general not knowing what to do or how to do it.

My personal opinion is that the greatest impediment to a fast evacuation is baggage fees. Somebody trying to save their roll-aboard from the overhead bin would cause a serious logjam in the aisle, but nobody is going to try to rescue their luggage from the cargo compartment. Get more baggage out of the passenger compartment.