Umbreon is a definite must run in dark decks with Ninja Boy.
Obviously, any deck with Hoopa EX or Shaymin EX should also run Ninja Boy.
Other than that, I can't think of any uses directed to a specific pokemon, but I'm sure there's got to be some pokemon out there that this is tailored to.
One suggestion I've seen is that if you're running, say, a Giratina EX and it's almost KO'd, you could swap in a non EX pokemon so you only lose 1 prize card instead of 2, and your Giratina could potentially be used again. Also, if you swapped in a pokemon that had less HP it would be KO'd and you would be able to move up a pokemon from your bench. For example, if Jolteon EX were about to KO your Giratina EX, you could then move a pokemon from your bench that would be better suited to handle Jolteon EX.
You could also swap out a pokemon at the last second to throw off your opponent. For example, if you have Gyarados in the active spot, you could power him up, wait for your opponent to power up his M Mewtwo or M Rayquaza and then swap in a Regice.
It's definitely a 1 of in most decks, the main draw back is that it's a supporter and there's no guarantee that you will absolutely be able to find it when you need it.
Obviously, any deck with Hoopa EX or Shaymin EX should also run Ninja Boy.
Other than that, I can't think of any uses directed to a specific pokemon, but I'm sure there's got to be some pokemon out there that this is tailored to.
One suggestion I've seen is that if you're running, say, a Giratina EX and it's almost KO'd, you could swap in a non EX pokemon so you only lose 1 prize card instead of 2, and your Giratina could potentially be used again. Also, if you swapped in a pokemon that had less HP it would be KO'd and you would be able to move up a pokemon from your bench. For example, if Jolteon EX were about to KO your Giratina EX, you could then move a pokemon from your bench that would be better suited to handle Jolteon EX.
You could also swap out a pokemon at the last second to throw off your opponent. For example, if you have Gyarados in the active spot, you could power him up, wait for your opponent to power up his M Mewtwo or M Rayquaza and then swap in a Regice.
It's definitely a 1 of in most decks, the main draw back is that it's a supporter and there's no guarantee that you will absolutely be able to find it when you need it.
