Well your point is well taken and I agree with the analogy to a considerable extent. I guess I was thinking about this more from a modern homebrewer's perspective than a historical one.
If I go about designing a recipe for a porter, I will approach it differently than I would a stout. I agree that what the BJCP defines as a robust porter may be difficult to distinguish from an American-style stout. There is a lot of overlap there. And if you submitted the same beer to both those categories in a well judged BJCP comp, you may score highly in both. I think your analogy is apt here.
However, if you take the rest of the sub-styles (brown and Baltic porter, and Irish, sweet, oatmeal, foreign, and imperial stouts) I see a lot of differences not only in flavor and body, but also gravity, IBU's, etc.
If the brewer doesn't care about BJCP distinctions/guidelines/competitions and feels like it is a porter and not a stout or vice versa, hey - who am I to say that is right or wrong. As a BJCP judge I am asked to adhere to and apply those distinctions when judging. I personally like to use them as an informational guideline when coming up with a new beer, but I don't feel constrained by them.