Ok, I agree with most everything you just said . Certainly you can fly at whatever airspeed you wish with the proper throttle and elevator trim. And I never said it allowed you to fly slower.

What I said was that you can fly slower and still have something in reserve for "OH *****!" with out waiting for the engine to respond. I will agree that 99% of the time this isn't needed though. And the vast majority of the time, it won't make any difference at all when it comes to learning to fly.

However, throttle response on most engines isn't linear, and there is a limit to the lowest speed you can idle. If you are flying from a short field with obstructions, a 7" pitch at 1,200 rpm vs 5" at 1,200 rpm can make a difference. Yes, I know you can slow down with the 7" prop by just feeding in a bit more elevator a bit earlier. And there are dozens of other tricks experienced pilots can use to bleed speed and hit a small field. But it's easier for many guys to with the lower pitch prop. Not all the time, but not never.