that sounds liek a great method. i shall have to try it out when someone asks me that question, or when i ask myself that question regarding something i do automatically.
on the other hand, i could see someone simply meaning 'why do you use qwerty' as 'what's the history of your using qwerty'. i wonder if this linguistic issue might cause us to conflate facts of the past with motivations. when i give a reason why i use qwerty historically it might just be 'well, that was what was available' whereas an answe to a 'why' question aimed at getting justifications might be 'well, i don't know if it's justified.'
perhaps this sheds some light onto the problem of confusing is with ought. we might confuse 'why is it' for 'why should we'.