I overall agree with you, but there are two benefits that I can think of to leaving your arm out there.
1. In combat, certain punches can lead to grabs and/or hooking the opponent. So a punch leading to a shoulder grab right into a sweep could work, and not require pulling the arm back. You are taking the risks of the reverse of the above though (more likely to be grabbed, less covered position, balance/mobility issues).
2. In training/practice, leaving it out so that your instructor can look at your fully extended position before your next punch.