This is just my opinion... but I think test cases without any assertions should either fail or give some warning. I've done this few times before.. I would write a test cases and thought it was passing then realize that I forgot to put any assertions. I should have been more careful, but it would've been nice if those cases failed instead. Test cases without any assertions doesn't make any sense anyway..
there are times where not making an assertion is valid. more important though, we would be breaking compatibility with all other frameworks