On his frequent business trips he buys a coachticket if the flight is shorter than six hours (it no doubt helps that he usually gets upgraded), and he travels on weekends.
Both Delta and US Airways have a three-tier redemption structure, offering seats at the basic level, a midtier level at around 40, 000 miles for a domestic coachticket and a full-fare level for as many as 60, 000 miles for domestic coach round trips.
Delta says it, like other airlines, offers all seats on planes to frequent fliers, just at higher mileage cost than the lowest redemption level, which is 25, 000 miles for a domestic coach round-trip ticket.