Dragonite getting ES doesn't totally explain Mence's drop I would say. Imo, it just made Nite a better CBer and an antiLead. Also, ScarfZ and ScarfLatias are two VERY VERY VERY different pokemons.
I think it has a lot to do with it. Just another option which makes Dragonite more unpredictable, and he is now easier to fit into teams because of Extremespeed(hits almost as hard as CB Scizor's Bullet Punch, mind you) revenge killing a decent amount of threats.
While Scarf-Z and ScarfLatias are two different pokemon, Scarf-Z doesn't bring anything to the table that Latias can't already accomplish. Latias has a much wider palette of useful resistances, which allows her to switch into Heatran, Zapdos, Gyarados, Infernape, and several others, while Porygon-Z can't really switch into anything. She is also faster, which allows her to revenge kill DD Salamence, a huge task for most teams. Her Draco Meteor is significantly stronger than his Tri Attack, despite the disparity between special attack(140x1.5=210, 80x2(adaptability, according to smogon index, just makes STAB multiplier x2)=160), and she also attains great coverage with the move. Finally, she has high base power, amazing neutral attacks like Surf to complement Draco Meteor, meaning she has less predicting to do. Sure, they are different pokemon, but if I am going to put a Scarfed special attacker on my team, I can tell you at the least it won't be Porygon-Z, as it doesn't bring anything to the team that, say, Heatran or Latias can't.