Combat Expertise allows you to sacrifice potential to hit, to get AC instead. You take a disadvantage to gain an advantage.
By attacking an empty square with no enemy in range, you get the advantage (extra AC) but completely negate the disadvantage; because there's no-one to hit. This, to me, feels like cheating a bit, you're taking advantage of the letter of the rules, rather than the intent of the rules.
1.Combat expertise affects the entire round, not just the character's own turn. It is possible to make attacks in a round in which you never move into range of an enemy.
2.When taking -9 to ab, the intent is generally to hold a position rather than to inflict damage, attacking the opponent is as much of a technicality as attacking an empty square would be.
3.The cost, to me, is a standard action, I could use it to set up a flank, to close with an enemy, or to execute a full run and exit the combat. There are many offensive things that can be done with a standard action that are being sacrificed.
4.'Raise AC with a chance to damage an enemy' is a more powerful action that 'Raise AC'. It is difficult to see the less powerful action being a cheat. More specifically, it is still sacrificing the ability to damage things with an attack, just more so.
5.Combat is supposed to be an abstraction of a continuous process, just because you do not have a chance to attack someone doesn't mean that you are not fighting with them in that space of time.
For a couple of examples.
Huge skeletons with spears, a coupe of formations, lets say 3x3.
1. Attack a square and move between them, abusing the attack grants me AC, I soak up the AOOs without taking damage and the rest of the party can charge freely, I am now in the middle of the group with high AC most mindlessly attack me as the closest enemy.
2. Full defence and move, Basically do exactly the same thing but with less AC and I don't threaten, so cannot take AoOs or provide flanking bonuses and I lose more HP.
3. Move and attack. Half of the AoOs hit me, I make a feeble attack at the end of the move, in order to protect against the counter-attack.
There is an ogre blocking the bridge, on the far side of the bridge is a team of archers. The bridge is rough terrain and takes 3 rounds to cross. bypassing the bridge takes longer and requires consistently high swim checks.
I engage the ogre while the party uses ranged attacks with cover against the archers.
1. I attack the ogre, the archers take -4 for firing into melee and I raise my AC to the point that they require 20s to hit. The ogre falls and I cross the bridge while attacking the arrows for AC increases. The benefits of no longer firing into melee are accounted for by the +4 and my abilities do not involve using my foe as cover. I enter melee with the archers and it is no longer an issue.
2. I cross the bridge using total defence. My AC is reduced specifically as a result of the ogre falling.
3. I double move over the bridge and the archers hit me with relative ease until I reach them, at which point my AC mysteriously increases by a dozen or so points...
reach[/url]]
An ogre appears and loses initiative, I cannot reach it with a move action.
1. I advance 1 move and attack a square, it can advance and attack my increased AC.
2. I delay and it gains initiative because I wasn't allowed to use my feat.
3. I advance and ready an action, it moves up and attacks me, I do nothing because I cannot reach it.
4. I reveal the lightyear-long whip I had the giants make for me and attack a square in a hobgoblin city a couple of thousand kilometres away because the ogre is invisible for some reason and I couldn't pinpoint its location and I can't justify picking one square over another...
Taking this any further would be bad for the game, so I am not going to reply any further on this topic unless explicitly requested.