To be fair, I think every series has a lot of episodes that would fail this test, some of which were excellent, like DS9’s “In the Pale Moonlight”, and “Far Beyond the Stars” or TNG’s “The Inner Light”, but if used to assess a series, I think this could be a good metric.
Indeed, “In the Pale Moonlight” is one I thought of which fails as well. I still think it makes a good measure to see how many episodes of a show pass/fail overall. Only to see if it’s really about the whole crew or mostly one character. (Arguably, early TNG comes really close to being Star Trek: Wesley while mid/late TNG comes close to Star Trek: Data.)