That's a very fair point. As Mr. Grégoire spoke to quite accurately, it is true that in cases of emergency evacuations, carriers do rely on able-bodied passengers to assist. One of the primary reasons is that in the case of an emergency, where time is of the essence, a flight attendant isn't going to obstruct the aisles to get to the disabled passenger first. People evacuate as quickly and in as orderly a fashion as possible, and people who remain in their seats are then aided by the flight attendants. If there are three disabled passengers left on a 100-seat aircraft, whether you've got three, four, or five flight attendants, that is going to have minimal impact on your ability to evacuate those couple of disabled passengers. Ideally, the able-bodied passengers around them provide assistance in those instances, as they are requested to do.
On June 20th, 2006. See this statement in context.