If we're talking 4e, an average AC for level 15 is 29 (level + 14). Average non-AC defense is 27. Average attack vs AC is +20 (level + 5), vs non-AC is +18. Those are monster numbers, but the PC numbers track pretty closely if you keep your ability scores, equipment, and feats up to date.And two-- it becomes much harder for DMs to guesstimate the numbers he needs to set up for encounters because they are just so big that it's hard to visualize. Most of us all know instinctually that at 1st level, ACs of 15 or 16 are average, an AC of 20 is really good, and a monster with an AC of 22 is a beast. But what are those numbers for level 15? Anyone know? I sure as heck don't (without pulling out the books and doing a lot of the math to get there.)
If we're talking 3e, it probably doesn't matter, at least not for PCs. At that level, monster attack bonuses far outstrip AC unless you're devoting all your resources to it.
After looking at the mathematical problems with 4e at the upper levels, I'm in the "ability increases are bad" camp. Also, I'd be down with reducing the impact of the ability bonuses. Castles & Crusades uses something like: 13-15: +1; 16-17: +2; 18: +3; which is a little less steep.