So could the existence of passenger service be a reason there are Class D's
around?

That's part of it, but there are many exceptions. Quincy, IL has
passenger service, as does Ottumwa, IA and Burlington, IA. All are
uncontrolled fields.

I used to assume that Class D existed because air traffic was once
heavier than it is today, and (as with all things government) newly-
useless facilities are slow to be closed. But now I'm not sure --
maybe they were *never* needed?

And there *are* examples of closed towers around. Galesburg, IL has
an abandoned control tower, for example.

My "event horizon" of GA is only 13 years -- perhaps someone who has
been flying longer (and doesn't have a vested interest in supporting
ATC) can comment on the history and usage of Class D towers?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"