Don't think of it as resetting propeller pitch. Think of it as resetting the propeller governor to let the engine run at redline RPM when at max throttle. Outside of engine failure situations, glide ratio effected by propeller blade angle is not a consideration.
Going to wide open throttle with the propeller governor set to, say, 2200 RPM does a few things:
- The power available is only that which WOT gives at 2200 RPM, maybe 85% of rated horsepower, when you may want all of it.
- WOT at low RPM raises Brake Mean Effective Pressure in the cylinders to very high levels and can bring on detonation if the conditions are right.
- Lastly if you go to WOT and then follow up with max propeller RPM applied suddenly, the propeller will typically overspeed past redline momentarily as the governor overshoots the RPM setting and has to catch up. Everybody for miles around can hear the result.
There are times when you may leave the prop RPM where it is, say if you are reducing power for an enroute descent, and when you get to the level off altitude, you're just going to bring manifold pressure gently back to its original cruise value. But for any descent where there is a likelihood of needing full power at the bottom of the descent, you ease the prop to max RPM after reducing throttle to ensure that full power is available as soon as throttle is advanced.