If you can get a Mad Hatter, Gracious Host down after turn five, you'll almost always get to draw one or two more cards, which can give you a big card advantage over your opponent, especially if your Flynn Rider forced them to discard from their hand
In a normal 60 card format, even as a 40/40, its killing anyone without regard to life gain in a single hit
You wont want to put it into every deck, as sacrificing a token and getting a 1-drop isnt the end all be all of MTG decks
[28-091R] Vorpal Bunny This ability performs a series of actions upon resolution, so you cannot Summons or use abilities after placing the top card of your deck into the Break Zone
Whenever one or more creatures you control become the target of an activated ability, draw a card
You get a 4/4 with trample, you spent 4 mana, you lost a 1/1, and your foes lose three cards from their hands