This is one thing that someone new to the BBoards may miss, because naturally, developers here want to "do it right" for 4.x. But in many ways, the older simpler 3.x style, while less powerful, offered an easier learning curve. Personally, if I was reviving the old training Bootcamps, I'd probably have the first couple exercises focus solely on building database-backed web apps in general, using the simpler ACS 3.x style, then use transforming those apps to use the more sophisticated OpenACS 4.x services as the next exercise. I think the same approach to buidling your own apps could work nicely as well, for folks new to OpenACS who just really want to something working quickly.
Of course, once you're already familiar with OpenACS 4.x than using its services doesn't slow you down, and is definitely the better way to go. But when porting old code, converting it cleanly to the 4.x style is probably always going to be more work than just shoe-horning the old stuff in with minimal changes, no matter how much of an OpenACS guru you are.