Being a CFI, you usually won't have the extra money to grab those 100 dollar hamburgers and you don't get paid for that time on the ground.
It costs the same as grabbing a Wendy's burger between flights at the home base, and the crew car has free gas. I'm not saying sit there for 3 hours, just shut down, grab a burger, talk about mistakes that were made and how to correct them on the flight home. Sometimes I'll treat my student to the food, more often I can't afford to- but stopping made my cross countries more enjoyable as a student, and it's not that hard to do.
Plus, how's spending 20 minutes on the ground between legs any different pay wise than getting back 20 minutes earlier? Unless your day is packed so full of students that you end up losing money, that is...