> Why would you use more spaces when and only when you have a longer space?
I've also wondered this. Referring to this mock-up[1], I have the same monospace text with single vs double spaces after sentences, and proportional font with single vs double. The usual argument is that #2 is better than #1 (for mono), but #3 is better than #4 (for proportional). It feels like an inconsistency in the preference, and (as you pointed out) the reasoning doesn't make sense.
People like to say that but it's never been explained to my satisfaction. Spaces are twice as wide in a monospace font. Why would you use more spaces when and only when you have a longer space?