Your logic makes perfect sense. I would have them finish the ceiling as well whilst there. Have you considered a metal ceiling? Not sure how they do things in your neck of the woods but I can get whats called "panel liner" locally. It is the same profile as the metal barn siding only made in a thinner gauge, thus reducing the cost. Once screwed into place your finished. I plan on using that for my ceiling and everything above 8'. My cost is about the same as buying drywall, but I am not paying anyone to finish or paint so it comes out about even. IF your paying someone it will be cheaper than using wallboard. Also, its a lot easier to install than hanging 4 x 12 sheetrock!