Also remember, Kirk was McCoy's best buddy. When Bones wasn't busy, he was hanging out because Kirk let him. Kirk didn't exactly run the tightest ship in the fleet, how many episodes ended with everyone falling around laughing? A lot of the time, Bones is palling around. I'm sure if there was something vital going on in Sick Bay, he'd be down taking care of it. Otherwise, he's a bit of a slacker. In Conscience of the King, he's more concerned with tying one on than listening to the XO voice concerns about the captain.
And yet, McCoy was an inch away from relieving Kirk of command in Obsession.