Diabolos' favor is fine if you're sitting around doing nothing and your PT needs MP, but if you're fighting something and you want a SMN to sit there with diabolos out and not use a BP at, because this will reset favor, then why bring SMN at all? If you're so hard pressed for MP bring a RDM or an extra BRD.
All of Diabolos' BP are pretty useless these days. Night Terror does more consistent damage than Holy Mist or Lunar Bay but that's not saying much. Dream Shroud doesn't do anything if you have an Acetic's Drink up, his Phalanx is useless, and let's not forget the glorified dispel move that can't KO an EP mob with over 50% hp.
