> what if there are 500 subtests in a loop and you don't want 500 failures to be
> registered for that test case?

Parametered tests have the same issue. In this case you simply don't use subtests
or test cases. On the other hand, the issue doesn't exist in most cases where you
iterate over three or four different cases.

> addMessage was just one suggestion.

It is quite orthogonal, actually, and could be added independently. Also, it is not clear how you would limit the addMessage to a subtest, rather than the whole test case.
We could re-use testtools' addDetail idea, although their content-type thing
is a bit heavyweight: I'd rather duck-type the API.

http://testtools.readthedocs.org/en/latest/for-test-authors.html#details