In reply to Andrew.

I completely agree with you, Andrew. It's precisely for the reasons you give that it's important to discuss the current suite of options with prospective clients, to help tease out and match with their current and future expectations. This is not necessarily straightforward when the final shape of some of those options is not currently knowable.

Justin has listed four potential types of theme. I do feel there's an opportunity here, where an upstream decision made now by WordPress—to endorse a specific approach to theming, if possible—has the potential to have a positive downstream effect: developers can focus a little more on building rather than choosing how to build, while future maintainers can have a little less trepidation when inheriting a site.