I belive you are correct -- they cannot deny a preboard request.

But I for one have been questioning the need for preboards for someone that can walk 300 yds to the gate, but not down the last 30 yds to the plane.

I guess I could always "fake" a bad leg and get one early -- although no real advantage as I do not like bulkhead and could not sit in exit row as a preboard. Only benefit is not having to wait in the jetway.

Of course, there are real needs for preboards and maybe your wife was one. As far as taking time to get on board -- usually that is not an issue unless you are an A1-25 boarder as the line queues in the jetway anyway after about 7 people get to the plane. Of course, may be standing on the ankle for 5-10 minutes which may be bad.