1. Obviously you fail to see my point that it slows it down. You focus 8 or 9 cards in your deck around getting and upholding a Dialga LV. X that could be speeding the deck up or helping another Pokemon in the deck out.
2. You don't one-shot Dusknoir, and Dialga G LV. X shuts your own body down, making you venerable. So you bring them to 70 with poison, then they one-shot you after LV. Xing and using Shadow Command. Bring another as they charge more on the bench, KO them, and activate the Stadium. And NIGHT SPIN. It one-shots Toxicroak then forces 3 energy on you. Dialga isn't going to help here. You aren't going to be wasting your time loading 3-4 energy on a Dialga.
3. How does that work?
4. Every spot in a deck counts. If you put a 2-2 Dialga G LV. X line in instead of 4 PlusPower, for example, you could lose your finals match by 10 damage. Sure, the Dialga may have been nice the one time you played vs. an Amphy-lock deck or saw that Mewtwo teched in, but there are still better options. A 1-1 Mismagius line can attack, search energy, disrupt your opponent and one-shot Mewtwo LV. X if you run enough tools. And that will be able to deal solid damage late game for no energy. That also gets around Mightyena who attacks to do that, whereas Dialga G sort of shuts down Toxicroak's body, letting Mightyena do that.