I haven't read the entire thread so forgive me if I say something that's already been said, but I got a gist of it from the first few pages. My thoughts on overmapping are that it's fine, as long as the rhythm is intuitive and consistent with the song. If you would make a rule that states "Hitcircles and slider starts should all be snapped to a beat in the song", then a map which is perfect and plays right, but has one note that doesn't follow a beat in the song - which is placed there for the sake of consistency - can't be ranked because it's overmapped. However, BATs would obviously not see the problem with this and rank it anyway, disregarding that rule because it would ruin an otherwise great map.
So the rule itself would have to be about as grey as the concept it addresses and the opinions on them. I do agree that there should be a rule on overmapping, but saying "Overmapping is forbidden in all cases" would severely restrict a mapper's creativity, and possibly even detract from the quality of a map, rather than add to it. Ranked maps are supposed to be enjoyable for all players, but "enjoyable" is something subjective, so the rules regarding ranked maps will have to allow room for subjectivity, or else all maps will feel the same and eventually bore the players, because of the restrictions on a mapper's creativity that objective rules will place.
I'd say a good rule would be to forbid excessive and unintuitive overmapping - this would include overdoing the distance spacing (i.e. random fullscreen jumps or those horrible jump-stream-jump-stream-jump maps), however if the overmapping fits the song and is consistent with the flow of the map, I don't see why it shouldn't get a map ranked just because there's an extra hit circle in there that doesn't represent any sound in the song.
In music there are things called ghost notes, which are notes that the musician plays very softly to be able to stay consistent in their playing, but that the listener doesn't hear. When a mapper places extra notes in a similar way to how ghost notes are used, I'd see no point in deeming it unrankable. Yes, whether this is "good" or not is subjective - but isn't that why maps need to go through the process of getting modded by several players, then a few BATs, and then still need 2 BATs to approve of the map for it to get ranked - with any other BATs who noticed a well-hidden and unacceptable mistake still having the authority to unrank it so said mistake can be fixed?
People are just making a way bigger deal out of this than it really is, and BATs shouldn't worry about mappers calling them on their decision, they aren't BATs for nothing.