I see the bench as the main chest exercise and the fly as more of an assistance exercise. Therefore I want to be sure that I can maintain the highest intensity on the bench and not worry about the fly as much. IMO If you pre-exhaust your chest, once you bench you will start using more shoulder and back muscles and will get less stimuli in the pecs. For the same reason I won't pre-exhaust my quads before squats or back before deads.
"Heaven and earth shall pass away, but my words shall not pass away." - Matthew 24:35
"But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, temperance: against such there is no law." - Galations 5:22-23