As the levels creep higher, pure AC becomes less and less meaningful...for the first iterative attack. Remember that all attacks have a flat 5% chance of hitting, regardless of AC....which is one reason that hit points play a factor in your survival. At high levels, as has been mentioned, miss chance effects and other defenses become more valuable...things like mirror image, displacement, blur and blink all become vitally important, as a 50% miss chance saves a lot of pain.
D&D's primary metaphor is 'eggs with hammers'. Most monsters cannot stand up to the punishment they can deliver...by design. Factor in the concept of attacking to aid, charging, haste, flanking and other effects, and you can see AC playing a factor, but not dominating, as it gets whitled down while BAB gets pumped up. Assuming standard D&D, healing magic is quick and plentiful, so high hit points really translates into consistent action: that is, how often you can continue to engage in combat without pulling back for safety. A high-HP fighter that can take an extra round to attack is more likely to win...and his chances increase dramatically if he has a cleric near by to heal him.
The situation then becomes one of either a) Don't get hit at all or b)survive a single volley, and then put them down before they can repeat the process. Spellcasters favor the former, melee fighters the latter. A wizard with a shield, blur and mirror image is just attempting to avoid taking pain altogether, while the fighter with fortified armor and a ring of blur is just going to turn around and show that rogue that he should have gotten away while he had the chance. At higher levels, you assume you'll be hit, and factor that it into your tactics.