I'm not suggesting that someone run seventy miles an hour, but running sixty will still get you very good mileage, and if your talking about a medium length run, that can make the difference of three or four hours which could mean unloading first thing, and first truck in the morning or fourth truck unloaded on a Friday mid day. That DOES make a world of difference in yearly in your pocket money. I normally did not have a problem getting to a delivery well ahead of the scheduled time.
There are enough clerks around that when lining up a truck for the company will say they need it by 17:00, and thats what dispatch will put in the load offer.
I will agree that there are some (very few) that don't want the freight there asap, but they are the exception not the rule.