Storm Raven said:
Actually, it does. There are many instances in the rules where a clear distinction is made between an ally and an enemy. This is one of them.
It doesn't answer the question, because the specific question is
why the distinction is made here, not
whether or not the distinction is made.
Going through the rules, with full knowledge that enemies and allies (as well as those who are neither) are treated differently under different situations is not a "house of cards". It is, rather, the only rational means by which one can determine why a rule makes that difference. Is it because of space concerns? No. If it were, they would be treated the same. Is is because you have to actively ignore a creature to prevent it from granting a flanking bonus to another creature? No, or you would have to actively ignore your ally. Is it because you are trying to defend equally against two threats that divide your attention? Obviously, yes.
Now, note that "defend equally" is the division of attention implied in the flanking rules. This implication includes within it
the idea that there can exist a house rule wherein attention is not divided equally. Is this a house of cards? Obviously not.
The only question then becomes IF you allow a house rule that allows for a different division of attention WHAT is the best way to model that division?
Claim that this is a "chain of illogic" if you like, but I am pretty certain that it holds up pretty well or you would have actually responded to that chain of reasoning.
As you say, "Just because you are not expecting an attack from an ally does not mean that you are actively ignoring them." You will note that I agree with this fully. Now, I say "
Ignoring an Opponent is the
title of a house rule, and does not mean that you are
actively ignoring him. It means, rather, that you are
not actively defending against him so that you may concentrate your defense elsewhere."
I have pointed out concrete problems implied by your ruling. Most obvious example: An enemy is anyone who attacks you. You do not know that your "ally" is an "ally" for certain. If you treat your "ally" as an ally, then he is in the same position as a foe that you are ignoring. Consequently, the same rules should apply to the "ally" if he attacks you as to the foe you are ignoring.
My house rule, or that of Primitive Screwhead, model this. Applying this to your house rule demonstrates either that (1) your house rule is less elegant (in that your rule needs to specify which type of foe it applies to, rather than applying to all foes equally) or (2) you'd have angry players if you tried to impliment your house rule and you tested it. This is a position that is easily testable. Have an NPC foe pretend to be an ally. Put him in flanking position. Proceed to give the PC an automatic coup-de-grace. Enjoy the rest of your evening dealing with the reaction of your players.
RC