Jeff Wilder said:
I keep saying "in order to gain the benefit, you must accept the penalties." And you keep saying, "No, you take the penalties when you gain the benefit."
Let me try to sort something out.
Hyp's argument seems to be that
through the act of holding two weapons, you have already
chosen to fight with TWF, whereas your argument seems to be that by holding weapons, you gain choices and can alternate between taking and not taking the TWF penalties (and correspondingly taking or not taking an extra attack)
while benefiting from TWD bonuses, irrespective of whether or not the penalties are applied. Isn't that a classic case of wanting to have your pie and eat it too?
I am astonished to see that someone would argue that a character was holding a weapon in his hand, benefiting from its defense, and threatening with it, but not wielding it and thus exempt from taking penalties from it. (If this is not your argument, then please say so, but it seems to be a succinct description of your position.)
Clearly, TWF is intended as a tradeoff. More attacks come at the penalty of decreased accuracy. This is a setup seen elsewhere - consider monk flurry, for example. Want to know what the ranger IMC does when he doesn't want TWF penalties? He only draws one weapon.
In case it isn't obvious, let me try to explain why Hyp asked his question. He needs to know if you are using IUS and considering it a weapon wielded simultaneously with the longsword. If not, great. It is indeed the simple situation you expected. Everything is perfectly normal. But neither are you considered to be armed with the IUS.
Since IUS isn't something you can physically be holding or not holding, it's not an issue that can be determined by asking if you are holding a weapon, and the "wielding" or "not wielding" distinction is clearly one of your choice. The same cannot be said for someone holding two weapons he is proficient in and claiming that he threatens with both and benefits from both. If he threatens with both, is he not clearly fighting with both? If he takes the benefits from both, should he not take the penalties from both? Is that not the underlying point of TWF?
Of course, your argument may have nothing whatsoever to do with the underlying purpose of TWF, and may be more of a "you can't explicitly prove that I need to take the TWF penalties when I benefit from a defending scimitar in my off hand, because the text is poorly written" argument. In that case, feel free to argue it here all you like; I'm not your DM, and I have no interest in proving it to you in a meticulous legal fashion. It may well be impossible due to poor wording, as you say.
However, your refusal to answer Hyp's question about your proposed situation appears to be indicative of your approach to this matter: considering each individual bonus and penalty piecemeal. As I explained above, the most plausible explanation for this is the desire to gain benefits from a situation without incurring associated penalties, when the two are intended to be concurrent.
This is why Hyp is so adamant and so careful in his responses here.