I disclose AC and other defenses, current and total HP and whatever else. For play-by-post, I just copy the monsters in the encounter to a thread for anyone who want to see them. Those who don't can just not open that thread. At the table, I don't give all that information out, but only because I don't want to inundate the players. I usually tell them "Every defense in this encounter is higher that X and less than Y" and most of the time they can get a good sense of whether they hit just from the die.
Primarily this is to speed things up, as mentioned above, so that people can determine their entire turn and not have to wait for my response or post complicated if-then statements.
Secondarily, I do this so it's clear to the players that figuring out those numbers is not intended to be part of the game. I'm not trying to test their math and memory skills, and frankly I don't enjoy people showing off their math and memory skills.
The way I figure it, most numbers on a monster equate to visible in-game things about it. Not that a high Strength monster will always have muscles, or that a high AC monster will always have heavy armor, but the characters can tell the equivalent of the numbers. If anyone is interested, I can explain how I rationalize that.