Offense is always effective, but defense is only as effective as the amount of firepower aimed your way. If you're an untouchable tower of steel on the battlefield, smart monsters will look for ways to bypass you and go after the squishy guy throwing fireballs instead. When that happens, your high AC does nobody any good; what your fellow PCs need from you is damage output.
This argument always seemed a little disingenuous to me. Sure, if you are not doing any damage then, yes, you can be ignored. But that is almost never the case. If you rush past the front line to go after the back ranks you are leaving your back open to the big guy with a sword that will kill you. You are essentially committing suicide to inflict maximum damage before you die. Is that really what your NPCs would do? Or is it a tactic that a game master uses because the individual pieces have no value?
If the bandits, orcs, goblins, or whatever has even the slightest sense of self preservation, they will leave an escape route. If they can't take out the big armored guy then taking out the wizard in the back will do them no good, they will still be dead. Now demons or undead might be a different story, and that's fun for the different challenge that they present to the PCs. But you shouldn't have all opponents disregard their own lives just to inflict a bit more attrition on the PC's resources.