If you look at riders riding on roads you are probably going to see a lot of "roadish" bikes. If you go to trail heads and look around you may find a lot of "MTBish" bikes, and as pointed out above if you go to interstates you will not see a lot of waterskiers. As I look back at most of the posts about this subject (many many other versions of the same thread) I see a consistant story. Those folks who ride roads by in large favor dropbars as the "only" practical alternative. Hard core MTB'ers on the other hand laugh at the concept of drop bars.
Changing Drop bar bikes to flats and flats to drops is one of the speciality niches that I do for folks on occasion and it appears to me that some riders just don't like certain setups. As a proponent of 50+ common sense I feel that rider comfort (mental as well as physical) is important. The biggest problem is that it is not cheap or easy to change back and forth between the two styles for comparison. It is not always easy to go to the LBS and find nearly equivalent bikes with different control groups to try. A few years ago, TREK sold the 1000 with both flat and drop bar control groups with virtually identical equipment fits. (other than those necessary to accomodate the change). At this time I don't know of any other identical bike to use for comparison.
I notice that there are many more flatbar road bikes in Europe where they are used as everyday commuters then there are here. Anyone here have any experience with that?