David, Phenidone does not just mimic the properties of Metol but has some unique properties of its own. Its use in the two Perfection formulas is to increase film speed and latitude. For example, the POTA formula (which uses only Phenidone as the developing agent) was developed by Marilyn Levy for the Photo Optics Technical Area (POTA) division of the U.S. Arms Electronics Command to be used to record atomic testing which needed a film/developer combination capable of recording a very large contrast range. This formula will give an 18 stop tonal range with most panchromatic emulsions. Ordinary film/developer combinations can only handle a range of 5 stops.
With high contrast films, such as Kodak Technical Pan film and document copy films, this developer produces a negative which preserves the normal pictorial contrast while retaining the film's extremely fine grain.