IMHO, the reason that comes up is because archers can defend themselves from any and all enemies anywhere at any time because they can make a ranged attack with no chance of hitting the target while melee characters cannot "set" themselves.
Thematically, this means that the archer standing in a field, or even an unarmed guy that just threw away his knife, can "set" his self against attacks, but the fully armored and armed knight standing in the same spot can't.
Logically, this means that the archer can "set" his self against enemies even if none are adjacent while the melee character never can.
Mechanically, they both spend the same action but only the ranged character can gain the benefit.
From a thematic, logical and mechanical standpoint, the rule makes no sense. It just seems an arbitrary decision.
I understand that Stephen Radney-MacFarland has ruled otherwise, and I get that my opinion is worthless, but I disagree with his position.
Ah, so it's a misunderstanding then.You can make a melee attack against nothing; swing at the air, etc. and gain the benefits of defensive fighting. But you must use your attack action to do so, meaning you'd have to forfeit your ability to make a standard/full attack that turn.
What you can't do, is just "turn on" Defensive Fighting (like a stance or something) and gain the benefit before you make an attack.
I think the key is the need to actually attack, not just be readying an attack.
So far as I can tell, you can always make a melee attack if you have a weapon. You can just attack a hypothesized invisible thing in a square next to you. If it's not there, well, too bad. You still make the attack, because if you could reveal for sure that there was nothing invisible in a square by intending to attack it and discovering that you aren't allowed, that would be overpowered. :)
Soooo... IF fighting defensively can only be used when taking the <attack action> and the <full-round attack action>, why isn't it stated clearly in the rules.
That "as a Standard Action: You can choose to fight defensively when attacking" sentence which is about as unclear as it gets has created about 40 pages of thread arguments about how to use fighting defensively and with what actions, special attacks and so on. Its time for an errata.
Until then I will always ask GMs I play with if I may use fighting defensively with actions that include any type of attack roll and take a standart or full-round-action.