Most people who actually know how to battle carry taunt. an attacking move is necessary in almost any set, especialy with Gengar's amazing SpA stat.
Plus, when Gengar gets hit, it gets hit hard. it doesnt survive, it just dies. you'll rarely ever get to use Pain Split.
P.S: I dont know if this is for WiFi or Showdown/Pokebattle etc., but in Ubers, Lugia would usually be sent out againt your Gengar and if it already has a sub up, 2 Shadow Balls ought to KO it
Hope I helped.