um no it has to be in the deck sorry to burst ur bubble but UNLESS they drop their own mewtwo its good tornadus needs it and so does regigigas
Reasoning? Don't just spout stuff off with out any real reason to why except for "They need it"
Yeah, Regigigas can block the recoil from his first attack with Eviolite and do 80. But guess what? Tornadus can do the same thing with out the extra tech and Hurricane's ability lets you power up a second Tornadus, Regigigas or whatever tech you are using (Terrakion for example)
Tornadus doesn't really need it either. As I mentioned before pretty much everyone is using Dynamotor Variants and all the attackers in those decks can OHKO Tornadus for free so Eviolite is useless.
What is the other deck that's getting a lot of play right now? Durant. How is Eviolite helpful against Durant?
Oh yeah, we'll see ReshiBoar and the occasional Tyram. Guess what? Even with Eviolite their main attacker can OHKO Tornadus. And you want damage to be on Regigigas so you can utilize raging hammer.
Mewtwo EX is going to OHKO Mewtwo EX regardless of the situation due to weakness. Eviolite doesn't help.
Celebi prime is too frail even with Eviolite so attaching it to him would be pretty much pointless. Same with Shaymin EX or any other tech you put into the deck like Victini 15
Eviolite is not needed in the deck because it defeats the purpose of the deck. The point of MTC/Purple Leaf is to get fast, and cheap prizes with Tornadus by hitting their weak basics T1 and catching in their weaker pokemon, basics and worn down Big Basics as the game progresses. Eviolite helps accomplish none of those goals.