I suspect that this POH was written during a time when retractable gears were not as common or reliable as today... at least that's what it seems like!
You're training to be a CFI and one of the things you'll be dealing with is the discrepancies between POHs, SOPs and common sense.
- In this case the POH is written for the early 1980's GA pilot, someone who may have trained on fixed gear types and is prone to forget the gear handle is there. Especially as 98% of the C172 fleet doesn't have a gear handle.
- A POH is different from an OM in that it should be adhered to, but it leaves a lot of room for the development of club or company specific SOPs that further specify how a particular type should be operated. While the note in the POH is clear, it describes something that Cessna labels as a best practice. It is not an absolute in my view. I would write up a SOP that covers a more common approach to gear use for this type.
- Common sense is that you should consider all aspects of the operation before deciding on a single course of action. One is the student's point of view: everything you do is copied by the student, so someone who is taught this practice on a C172RG will no doubt carry out the same procedure on a higher drag retractable later on. Performance wise, pulling the gear up once you can no longer set down on the runway improves your climb performance and cuts down on your fuel use. As for the maintenance issue, good of you to think of it, but as a pilot, think about the fragility of your own neck before considering the depth of the pockets of your employer. Unless you get a direct order to follow these instructions to cut down on maintenance costs, I would pick the safest option for myself first.
- Another thing to think about: how long does it take for the gear to travel up and down? I have not flown a C172RG and don't know the times, but if it's a slow system and you're flying short circuits... the timing may mean that it's easier just to leave the gear down, but I would always clearly state the deviation from 'normal' during the flight. So "Gear up... non-standard, leaving it down" and "Gear down... non-standard, it's already down" at the appropriate times.
As a CFI, get used to thinking of all the angles and all the factors that influence aircraft performance and human performance. The fact that you're using an 'interesting' type for your training helps, in that it sets you up for dealing with all the other strange POHs out there!