Certainly, there are *some* cases where AC does very little. Most examples that I hear, however, aren't true. DQ2 epic, for example. Despite having a 95+ AC in there, she still connects a fair amount. However... a "fair amount" is a far cry from "every swing". My AC build has about half the HP of my barb, but survives just as well... half (or so) of her swings miss.
Quikster, you're dead on about the grind, and that may be an important point to this perception problem. With no facts to back it up, I'd guess that more than half of those who try AC builds give up the chase before getting anywhere close to their desired gear. An AC build is not for someone who lacks the dedication required to properly equip it.