If you find yourself convincing a random stranger with no skin in your game that your particular decision is right, then it's likely that your decision is shit. Or so you are.
For a close analog of the random stranger, such as an ex-colleague or distant relative, this argument stands as well.
There are three kinds of users: those who read the manuals, those who don't read the manuals and those who read the manuals and then still do weird shit.
Building software that suits the needs of one group is hard. Building software that fits all of them is logically impossible.
When person says something, it's typically safe to assume that she has reasons to say that. Everything else (i.e. that she believes that; that she's telling the truth; that she wants to make certain impression; that she wants to be heard; that's she's competent to make judgement etc) is to be verified separately.
Given a choice between spending a lovely evening with family and friends or spending same evening in an exhaustive Internet flamewar about human rationality, what would a truly rational man do?
Do you know that in Scheme it is legal to write
(set! car cdr)
?