> 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.
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.
[1] https://imgur.com/a/nozuw