You may be right in that the argument comes more from a concern with how a broader public relates to the term of ´rational´ than how it is used in the mentioned disciplines.

On the other hand I feel that the broader public is relevant here. LessWrong isn´t that small a community and I suspect people have quite some emotional attachment to this place, as they use it as a guide to alter their thinking. By calling all things that are usefull in this way 'rational' I think you'd be confusing the term. It could lead to rationality turning into a generic substitute for 'good' or 'decent'. To me, that seems harmfull to an agenda of improving people's rational thinking.