@ShivaX read (and this makes sense) that with the really big, dangerous melee threats your high AC might or might not protect against that first attack in a round, but it helps a ton against second, third, etc. attacks (which all lose a stacking -5 off the base attack). So in the above, that Owlbear may only need a 5 to hit you, he needs a 10, then a 15, then if he has a 4th attack, it needs a natural 20 to hit. So you save yourself quite a bit of pain by having as high an AC as possible.