That the consultants' personal time is high on the scale is why they pay for the flex-fare tickets. So you miss an 8pm flight for one reason or another - it happens, so "don't miss the flight" isn't a valid retort.
Do I feel better taken care of if I can just take the following morning's flight at no additional cost or hassle, or if I now need to contact someone at Amex Global Business Travel, have them try to get me on a flight, have another expense, potentially not be able to get on the next flight or the one after that, etc.
The company can start with heavily-discounted restricted tickets and eat them when a consultant needs to extend the trip. Today it's even easier because airlines have eliminated change fees - so essentially any ticket can be canceled and those funds credited to a future flight. It's all still "no cost or hassle" to the consultant since they aren't paying for the flights to begin with.
Do I feel better taken care of if I can just take the following morning's flight at no additional cost or hassle, or if I now need to contact someone at Amex Global Business Travel, have them try to get me on a flight, have another expense, potentially not be able to get on the next flight or the one after that, etc.