The good thing Dialga has over Groudon is the Toxic immunity which allows Dialga to switch in on many Defensive pokemon that might use a surprise Toxic, such as Forretress or Ferrothorn. Groudon is crippled by Toxic on the switch. Kyurem is really weak to hazards and Forretress can set up on it for free.
<&GreatSage> if nothing else it's a "trickroom teaches pokemon" thread
<&GreatSage> which is reason enough to keep it
<@Colonel_M> I once heard Jibaku considered Scarf Gyarados for his Ubers team
<syrim> Trickroom the ultimate synguuest