One of the places where there is a lot of push-back against using the dash is this:
...Q The car on the left -- what color was it?
...Q The restaurant where you were eating -- is that one where you often eat?
If you think of the first dash rule as "broken sentence structure" rather than just as an "interruption," this dash makes a lot of sense. The beginning part of the sentence is a sentence that got started that did not get finished.
The other clue is that there is always something in the second sentence that refers back to the "false start." A comma just doesn't work. There is no comma rule that works.
Happy punctuating!
Margie