This is an awesome discussion. Well worth having.
Cool. Let it not be said that I never did anything good. :)
I think you and others have the right idea on why not to do subthemes. If you plan to build your theme and never upgrade the theme, it makes sense. There have been times when I want to upgrade the Blueprint theme and it has been so nice to just drop in the new version without touching my subtheme. Also, I had to upgrade a Drupal 5 site to Drupal 6 and having a subtheme seemed to make that go smoother/quicker.
OK, this makes sense.
Andy, your point is also *very* well taken. I have always approached themes are just a wrapper for HTML/CSS/JS and never really delved into the functionality aspect. I always figured that the majority of that stuff was for people using the theme "out of the box" and wanting to make changes without coding. But I think in D7 especially, more functionality is being opened up for coders also.
Thanks for the feedback everyone.
F