It is indeed rare; I happen to have one on record during my time in the game. A long while ago now.
I did float this as a conceptual possibility though, as every idea proposal really should factor in as many possibilities as possible to evaluate its viability.
While I personally would suggest leaving the strongest for the flip, what about the obvious sandbagger who puts out a team of unlevelled 1-stars? And everyone in the attacking alliance are point chasers? Meaning no-one hits this defence? Is this defence who conceded 0 points to the opposition as good as the defence which is the latest 5*s LB2, fully emblemed up with level 30 troops which no-one hit?
Again, an unlikely scenario but a possible one. Both teams ‘prevent’ a flip. But to rate them equally as good? Maybe for some. Personally I just think ‘not applicable’ is best for a rating in such a case. Because to say these teams preventing a flip, because no-one attacked it is as good as a team which is attacked and conceded 0 points is… odd. To me, at least.
Interpretive issues aside, this discussion reminds me of this idea: