Hmm... good points. Knowing the option is there easily enough is good as well. I have to say doctests are really straightforward for our own controllers. Part of the reason is twofold though. One is to develop tests for our own, the second is to (eventually) develop tests for web2py. I see there are some tests included as is, but as more and more people take notice of it, there will be more questions about code coverage.
For example some of the code issues that have limodou (hope I spelled your name right and sorry if I did not!) and I think you (voltron) brought up would be good candidates for tests as well. In my extremely limited experience and reading with test driven development, if there is an unexpected error or even a confirmed bug, make a test that is run so that it doesn't come back unexpectedly. Again that's just based on very limited exposure at this point.
Tests separated out are also partially useful because PyDev includes the ability to do unit test from within Eclipse as well. That still requires some investigation though.