Multiple Stage 2 strategies need to be looked at with a lot of scepticism.
The Ditto confused me at first, but there's actually numerous uses for it. They all center around the fact that you can put a Pokemon on the field with Energies and other things on it before you have to dedicate to what exactly it is. It threatens a lot of things by being there, but it leaves your options open until you want to choose a path.
For example, in Eelektrik decks, you can just play down a Ditto, and then next turn throw a Tynamo and an Eelektrik on top of it and you never had to deal with a 40 HP liability on your field. Or, if your opponent has a Mewtwo in play with no Energy on it, you can put some Energy on your Ditto. Then next turn, power it up fully, turn it into Mewtwo, Catcher theirs and X-Ball for two prizes. But you can also turn it into a Zekrom, Zekrom EX, Raikou EX, Bouffalant, whatever you want.
"But what if they Catcher and KO Ditto and its silly 70 HP?"
Then they used a Catcher on Ditto, leaving alone any Eelektriks or powered up attackers such as Zekrom or your EXes. It is only worth 1 prize, and they only have 4 Catchers to work with to begin with.
Now whether decks will have enough space for this thing is a large question mark, but I see no reason not to play at least one Ditto in theory.