There are 3 reasons I can think of to play this card. 1. You are maxed out on Indomitables -- in order for this to trigger during the villain phase, you must have already defended against a villain attack, in which case Indomitable does the same thing. 2. You are playing Thor or Valkyrie, who have the ability to engage minions during the player phase -- a decent idea, especially since readying is exactly what you might need to defeat the new minion. 3. You are Black Widow -- with her Preparation synergies. I think this is the option it will be played with most since the value of a 2-ER ready is not super high, but the extra value from the Preparation trait makes it worth it.