here is something I took out of my log it could be me asking the questions wrong or it is the NPC you be the Judge
(19:27:16) Nyshyn Klannarr says: You want something from me, do you?
(19:28:09) Soylentshadow says: what do you have?
Put yourself in the shoes of Nyshyn for a moment. She's busy examining artifacts and ruins with a specific purpose in mind. [ Once you dig into the Yliakum settings a little, you'll realize that what she does is crucial to her existance, even her sister follows the exact same path. ]
You walk up, disrupt her tedious work and proceed to demand she tell you what she has available. I doubt she'll start giving you a breakdown of the contents in her inventory or give you the choice of what you shall do in order to satisfy *your* desires. Would it make sense that she should respond to the last question above if you were to respond with "maybe"?
Perhaps you might consider that in fact its your character that might serve her, assuming you provide a clear intention that you are interested. Imagine the nightmare, given the dialog above, if she *did* only respond to the key phrase "what do you have?".
If i ask you "do you understand what i'm trying to say with this comment?" and you respond something else than yes or no, you can expect me to follow your response, i'm sentient (although thats debateable). If you need an NPC to carry out a conversation about subject X or Y, you have to do it programmatically. Just collecting a primitive boolean response is task enough, now consider writing an AI that would use fuzzy logic to determine what you are attempting to say. Each NPC would then require its own fuzzy logic. [ Its been suggested that a super-computer -may- reach a state close to human intelligence around 2020. You would need 100's of super-computers to provide 100's of sentient NPC's ]
If an NPC asks you a specific question where a boolean value yes or no is expected, the best bet is to respond with a boolean value. Why? because they aren't there to serve you. You may think they are but thats an illusion.