My take is that per diem shouldn't be comp because travel frequency and destination are variable. The employer has the authority to ultimately determine if travel is required. Why should the employee take the risk?

If it is built into my base comp then each time a trip is proposed everyone takes one step back. The "volunteer" gets the honor of higher costs to eat on the road than the office mate remaining in the cube. Or I volunteer for the trip to Tulsa rather than NYC

A per diem (ideally with COLA for location) assigns the employee some limited risk that reimbursement by receipt does not. It is a good balance of risk to authority. I can manage my travel meals to some extent but food in the terminal will always cost more and brown bagging isn't always viable based on location, trip length or whether hotel has refrig