I am looking for a rule-wise explanation of the logic as to why your deflection bonus (e.g. a shield) is counted in your flat-footed AC.
From what I understand, being flat-footed means you didn't see the attack coming, and are unable to react to it, so only your passive armor counts (meaning the opponent still has to hit 'hard enough' to actually pierce your protections).
In that context, how would you be able to parry? You can't dodge, you should not be able to parry as well.
I realize a shield can be useful if you can't dodge because you 'just have to raise it'; The problem is not every flat-footed situation should allow for a shield to be used.
Pushing things to the extreme: consider an invisible character sneaking to a guard. The combat is not started, the guard is unaware of any danger. The character is behind the guard, unseen, unheard, and ready to hit. Why would that guard's shield be taken into accout? Why would his armor even be? The character should be able to stick its dagger in the guard's neck (or any other not-armor-covered part) and deal a good amout of damage without having much armor to go through.
If I'm not mistaken, according to the rules that guard should be flat-footed. This means if the character attacks the guard, the rules consider he is able to: sense the attack, turn around, deflect the attack with his shield.
From the logic of that context, I'd say that guard should be considered defenseless, shouldn't he?
My table had trouble with a player recently contesting the logic of that rule (and honestly I can't blame him), and we had trouble making him accept that yes, his paladin's shield was useful against that ogre's enormous club.
Best Answer
First, shields don’t give deflection bonuses. They give shield bonuses. The difference is not so important for discussing flat-footed targets, but touch attacks ignore shield bonuses (and don’t ignore deflection bonuses), so the difference is pretty important when someone is trying to touch someone else.
Second, abstraction is a major part of the game. Both shields themselves and the flat-footed status are heavily abstracted, and cover myriad different things with singular mechanics. Abstraction is necessary for any game, to simplify the rules and keep the game moving, but by definition they must come at the loss of verisimilitude. By covering many different things with one rule, you accept that some of those things don’t really quite “fit” the rule precisely.
So, for shields: there is no concept of “active” shields and “passive” shields, so a giant tower shield, which is effectively a wall that you carry around, grants the same type of bonus as a simple light wooden shield. Also, note that the “buckler” appears to be used more like what the real world called a “targe,” strapped to the arm, rather than actively in the hand as true bucklers were. True bucklers do not appear in the game.
And flat-footed or losing Dexterity to AC (which are separate things!) covers lots of different situations:
Being attacked by those you cannot or do not perceive, such as invisible opponents or being caught by surprise.
Being distracted by other things, for example when you are climbing or balancing on something.
The result of various effects, such as being stunned.
Note that these are all very different, but use pretty unified mechanics (some are flat-footed and some just lose Dex to AC, but since we are talking about AC and ignoring uncanny dodge, I’m ignoring that distinction here since it doesn’t really come into play).
So someone with a shield and flat-footed might have their arms completely limp at their side, and the shield just dangling.
Or they could be holding up a huge wall, just not really certain where an attack is coming from.
Both of these situations use exact same mechanics. As such, keeping the shield bonus to AC is a compromise—chosen to keep the game simple and to make shields a little more valuable than they otherwise would be. In the first case where the shield bonus maybe shouldn’t really be in play, the bonus is small, but losing the large, expensive bonus of a tower shield in the latter situation is just wrong. So getting the latter situation right was favored here.