Adam Warlock's stat line is not amazing, so readying him may not seem all that great. But you probably want to be using Battle Mage every turn anyway since it's just innately a good value (even before you get your Mystic Senses online), so you may as well have the Cape to get a little extra value. And then since you're expecting to get an extra ready every turn, upgrades like Heroic Intuition and Armored Vest start looking better.

Plus, who doesn't want a cape?!