I've learned it's easier to give people what they want than to educate them. I supply them because people coming from the glazed-pot Kamado world think they're very important. Glazed pot Kamados are basically un-insulated and when the ambient temperature changes/drops the temperature can crash. Having a heat deflector reflecting heat back into the firebox gives these grills thermal mass/ a heat sink to help stabilize temps during these ambient temperature swings.
Because KK's are so well insulated, this is not a factor. In fact, the heat deflector requires you to burn more fuel, creating more airflow and more evaporation and less retained moisture in your meat. I suggest, preheating the grill empty and then putting foil on the lower grate the size and the area you want to be indirect. Put your drip pan on top of the foil, install the main grill and put your meat above the area with the foil. Put the upper grill on top of that and more meat. You're off to the races. The foil is much higher above the charcoal than where the heat deflectors sit and will not trap/reflect as much heat back into the firebox.
That being said I have not use the heat deflector in one of my grills for probably 10 years.