I finally knew why Giratina, Palkia or Heatran were given movesets like this now

I believe that it's because those movesets make them much more dangerous in pvp. Dragonbreath is a much more dangerous move than Dragon tail, and Draco Meteor or Fire Blast's long cooldown won't even matter here.

The only thing that sets Fire Blast and Overheat apart is fire blast's lower power and longer cooldown, the latter is offset by the pvp's altered move behavior. So while heatran is still a bit inferior than Moltres in terms of dps and ground resistances, the margin is pulled away by a noticable grip.

As for Giratina if you face one and you don't know how to properly shield its moves you are in big trouble, because its attacks get spammed often and are seldomly resisted.

Dialga and Palkia will literally start occupying masters league once they get released.