While the purpose of Giratina's Ability is pretty blatant, I don't see why everyone thinks this is the end of Greninja and Trevenant (and Yanmega). This reminds me of that Marowak card that everyone said was the answer to Seismitoad. Did that see any play? (Though, to be fair, Giratina is a Basic and Marowak is a Stage 1.) And there's also the matter of making room for at least one Giratina in your deck, just to tech for a specific deck that maybe you'll have a match against, which can be countered by a Silent Lab. (If one can make room for a Giratina, a Greninja player can make room for a Silent Lab.)
Who knows. Maybe it'll see play (if it's released outside of Japan). I just don't think it's amazing.