This ability gives Black Imu strong late-game presence and excellent board-control potential
The way they're played is to stall with Chansey while attaching a Fire Energy to Moltres every turn and then scooping Chansey up when it gets too much damage, using Wildfire, putting Chansey back down, scooping Moltres up, putting Chansey out, attaching Fire Energy to Moltres, etc
Featuring Cynthia, one of the Pokmon franchises most iconic female characters, the cards appeal is further amplified
Since it's a mana cheap instant, you can do this in response to targeted or sweeping removal to get cards, as a sacrifice or it can send something to your graveyard about to get exiled by Swords to Plowshares, or to get value out of a chump blocker about to die anyway
The decks that already run Worship in their sideboards probably arent going to switch over to a less consistent copy of the effect, even if Serra is more versatile