>Laughed hard when he said that thing about Mewtwo sucking in Eels ^_^
1 terrakion is game changing vs some of the most played decks (eels and Drakrai) it is possible to trade 1 prize for 2 using retaliate (and I know you understand this) The mechanic of this deck would more or less like to continue what it is doing (drudigon and amoongus) with the chance of using terrakion if the opprotunity came up. It is used as a scare tactic and also for the revenge, and if you wanted, you could make this into a revenge kill deck with a splash of drud and amoon, but I think they want the other way with having a splash of terrakion. Terrakion might not die easily, but there are a lot of cards which can help KO them easily (like darkrai wearing him down on the bench with night spear, or a zekrom EX catchering him then using his 150 attack, garchomp with 2 altarias on the bench, or even hyrdreigon using the 140 attack). I love terrakion, but I like quads of him, with a splash of Terrakion EX.
Anyways, onto the Bianca. I love Bianca, maybe not the hat, but she's a sweet girl and an amazing draw card... for certain decks. In my Ho Oh deck (which is one of my fave decks to play) Bianca shines brightly, as I keep using ultraball and junipers to quickly get things into the discard. I have plenty of items which i use constantly so I can have the edge on the opponent, and I dont care if items get sent to the discard, as my pokemon are strong by themselves. Bianca refreshes my hand beautifully so that I have a nice 5 card draw (sometimes 6!). But in other decks, one which uses rare candies / devolution spray / etc etc etc) I need to make sure I have certain cards in my hand waiting for a combo. I can't just ultraball my handsize down, or I risk losing an important rarecandy, or a clutch catcher. And the only way to get them back is Sableye, which sadly doesnt run in my deck. Thats when Cheren comes in. He's the bro who's got your back and understands that sometimes, a fresh hand isn't what is needed. For those decks, I tend to run 3 Juniper (OMG IM CRAZY) 4 N, and 4 cheren (plus random recievers) due to juniper sometimes being the worst thing to happen to my hand. N works since I dont discard the cards, which is a safer bet.
Now for the reason why you would want more cards like that in your hand... if you drop an amoongus and have the clutch going on with the druddigon, and your opponent trusts in the heart of the cards and draws a switch... you might need get another amoongus down STAT! (this never happens in darkrai though, as I have yet to see someone using a switch) If you could have 1 energy a turn, that would be great, right? drop an energy, and continue with the turn, what if you dont have one... sucks right? Well what happens when you have 5 energies in your hand, and you know you need to drop these guys each turn. bianca wouldnt help too much, and you could risk a N or Juniper if you are needing anything OTHER THAN energies, but cheren says it's okay bro, I'll just get you some more cards in the hand, so you can keep dropping energies and making it rain on dem Ho-Ohs.
Anyways, hope this helps in the explanation process :3