I've seen a fair amount of wheel-chair-using/mobility-limited passengers fly on flights in India where the airline provided a good deal of help to get the passenger to/from their seat, but it never was such full-time assistance that I'd count on an FA focusing on the person in the event of an emergency evacuation.

Much the same goes for minors traveling in the US as unaccompanied minors.

Speaking of IT again, do they always want the mobility-limited/disabled passengers to be in the aisle seat, or is it just sometimes? It's neither guaranteed nor likely that an FA is going to be the one first assisting such passenger in the event of an evacuation. Also, such a passenger is more likely to end up "getting in the way" when in an aisle seat or otherwise blocking an aisle access during an emergency evacuation; and so it seems like positioning of such passengers is done for the convenience of the airline and/or disabled passenger than for reasons of facilitating rapid evacuation.