In addition to overlooking the extension of the landing gear, leaving it down for circuits assures that a gear up on the next landing is impossible. The 172RG/182RG landing gear systems are excellent, but not perfect. (They are different from the 177RG, and some 210's gear systems).
That being said, from a pilot training perspective, learning to not operate the landing gear, when normally it would be operated is not ideal either. I am aware that a smaller airline which operated a mixed fleet fixed gear (Twin Otters) and retractables, had DHC install landing gear selectors in the fixed gear Twin Otters, and put that action on the checklists, just so it became muscle memory for the pilots to operate the landing gear and check position for ever landing, even if the Twin Otter gear was always down!