Mapache
Explorer
I put up the all monsters' defenses on a whiteboard (along with the players', all in initiative order). It really speeds up combat for everyone to be able to check if their attacks hit without having me consult my notes. Additionally, I think being open about monster defenses makes for more interesting choices about what defense to target. If you don't know the defenses, and you don't happen to have that monster memorized, then you're not going to find out what they are over the course of a few attacks during a fight, so you're running blind and just picking your best power. Informed choices make for better gameplay. (And if you want a simulationist justification, the players knowing the defense numbers represents their characters being capable of sizing up opponents, being the experienced combatants as they are.)