We were not asked for airline I.D. at check-in last time we stayed on my brother's airline employee discount. He was, however, with us at check-in. The room was in his name.
Note that if you reserve a room with the airline discount and don't follow with the letter providing proof of employment, they don't cancel the room. They simply change the reservation to rack rate!
We found this out when we booked a room once, then changed our mind, so didn't send in the letter. Later, we did decide to book, sent in the letter, and found that we had two reservations -- one at the airline discount rate, and one at rack rate. We cancelled the rack rate one -- no harm, no foul. But, it would have been a problem had we not found out about it. We'd have lost our one-night deposit.