Look at your local building codes before questioning me, I know full well that in my area that vapor barrier and a minimum of R40 insulation is a code requirement. As well, a ceiling that prohibits flame spread is also a requirement. It doesn't have to be drywall, but something has to be there, regardless of the finished product. Drywall is the least expensive product, hence why it is used in 99.9% of applications.

Look at your local building codes, in the majority of areas, insulation is required in the attic space, plus fire code does not permit things like wood ceilings without a drywall backer.

It's not really a drop ceiling if it's the only ceiling, is it?

As far as I know, it's not legal anywhere to make a drop ceiling into THE ceiling, you still need drywall covering the framing.