I expect this is mainly a disagreement about definitions? Many people think of "rationality" as referring to system-2 type thinking specifically, which isn't universally applicable and wouldn't actually be the best approach in many situations. Whereas the LessWrong definition is that Rationality is Systematized Winning, which may call for intuition and system-1 at times, depending on what the best approach actually is. With that definition given, I don't think selling "rationality" to people is something that needs to be done - they might then start dismissing the particular rationality technique you're trying to get them to use as "irrational", but presumably you're ready for that argument.
So you mean the person who I was talking to had a different definition of rationality? I wonder whether most people feel the definition is quite subjective? That would actually be quite troubling when you think of it.
I actually instensely dislike that way of expressing it, mainly because argumentative competitiveness is a massive enemy of rationality. For me rationality probably comes down to instrumental truthiness :-)
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