I suppose I'll take up a devil's advocate position.

First off, there's the ratchet effect, which has been discussed in painful clarity. In Piggy's case, the specializing in AC was effectively worse than useless in the metagame, as the end result was that the warblade ended up just as vulnerable, and his allies were more vulnerable. In PO it's important to consider the effect of your efforts on your DM; yes, this is very messy, but it's worth analyzing different scenarios for robustness.

Of course, as was previously discussed, the DM can swap feats and use granted treasure to make the baddie more effective than his SRD entry indicates. IF this is used not as a direct response to a high AC, and more as a response to overall party optimization, it's not necessarily a specific ratchet effect. For instance, you can roll random treasure for a Fire Giant, or you can just give him ~5800gp worth of treasure. You can let him keep his feats that he's never going to use, or you can give him better ones. Just a +1 Large Greatsword and Law Devotion (Fire Giants are "often lawful evil") and Weapon Focus is going to give another +5. Devious DMs may provide buffing allies and flanking mooks which don't add much to the XP total, but could add quite a bit to the chance Mr. Fire Giant hits. This is all situational, and it obviously doesn't happen in every game, but it can happen.

...Which leads to when it does happen: Boss Fights. These are likely your most dangerous, difficult fights. They're often many CRs higher than normal, and they're often optimized above normal. One could argue that this is when you need your protection the most; this means that if the boss (or more likely his dragon) even targets AC, you need to be able to make that attack have a good chance of whiffing. That target number is going to be far higher than normal. Say we have that same Fire Giant, except since he's a boss fight, he's a Fire Giant Fighter 2 with the elite array, more lewt, and dungeoncrasher. Now he has a Large +1 Magebane Spiked Chain. His feat loadout now includes Combat Reflexes, Knockback, and Knockdown along with Law Devotion. You're going to want more than 35 AC in this scenario.

Yes, you can make it so that even this particular Boss-Brute-off-the-top-of-my-head has a hard time hitting you. The question is, was whatever you just came up with more cost effective than just grabbing some miss chance items and investing the rest in being able to kill the brute first?