> I prefer no spaces around em-dashes for the entirely subjective reason that I find it aesthetically pleasing. It boggles my mind that other people look at an elegant em-dash and think, "Yes... with spaces... now it looks right."

I think it's because with no spaces, an em-dash looks like it's connecting two words into a single word.

Name another phrase-level punctuation mark (period, comma, semicolon, colon, quote, question mark, ellipses, parentheses) which isn't connected to a space. There's a reason for that.

[Hyphens, apostrophes, slashes, etc. are word-level.]