Well put it made me understand Signal Beams purpose.Since both Shadow Ball and Psychic from Alakazam 2HKO Rotom, its not like Shadow Ball is needed to hit ghosts. It's essentially the same story with Dusknoir. Psychic + Fighting + Bug is perfect coverage just like Psychic + Fighting + Ghost is, so you can't really argue that Shadow Ball gives better coverage. Overall, I think being able to OHKO Celebi and the ability to finish off weakened Dark types without risking 30% death makes Signal Beam the superior choice.
~Star Fox