Yes- and also an accountability thing. If a pwBPD never gave a hard "yes" or "no", then whatever you claim they have agreed to is arguable. So I suppose for them, the more blurred the line the more leeway there will be for future arguments.
OMG yes. 100%. dBPDh almost never gives me a yes or no because (he has confirmed this) he feels like I'm going to hold him to it later and he's never allowed to change his mind. That's not true, of course, but it is exactly how he thinks things will play out.
He also seems to have a very difficult time describing any ambiguity he might be feeling. If I ask him a question and the answer is not a yes or a no, he's not sure what to say. I attribute this to the fact that he mostly sorts things into black and white categories in life, and doesn't know what to do with grey. So he sits with it and agonizes over it until he finally decides to reluctantly put it in either a black box or white box. Once it's sorted into black or white, though, it still causes him anxiety because I think deep down he knows it's not the "correct" place for it, because it's grey. My H is high functioning though, so that last part may not apply to lower functioning BP's who could remain unaware of grey areas at all.
Clear as mud? Lol. Many of our talks, arguments, etc. are riddled with very, very, VERY long silences on his part because he really has no clue what to say.