True, but it does happen. I almost lost a gig once because a director came into my studio and was baffled that I didn't compose with Pro Tools (yes, believe it or not!). It also helps to be somewhat fluent with Pro Tools, as most editors request session files in that format.
Ok then ... well, whatever the client wants, right? As we are all service providers, we have to take the demand into account, of course. But still, YOU are the professional in that area, NOT HIM! So, talk him out of his belief :D