It wouldn't matter if you synchronised the carriers so that the phase relationship was identical as it left the antenna. There would be beat notes because the phase relationship at the aircraft would change according to its location and speed. Offset carrier was the only way to go. Older 25khz rx's have a special circuit to deal with the beat notes created (climax squelch circuits) I guess the newer 8.33 units don't. Although if the system is re-introduced they will need it.