I mentioned that as an advantage in one of my other comments. An advantage both ways, because it depends on preference. I have the same preference as hansvm: I would rather see the dangling dash at the end of the line, so I prefer putting spaces around the dashes. Having the entire word-dash-word structure move to the next line feels ugly to me. As with most things, de gustibus non est disputandum. (And also, quidquid Latine dictum sit altum videtur).

It's the dangling dash at the beginning of the line that gets me. I see a lot of word break algorithms, including the one WebKit (and I suspect Blink) uses, which are happy to break "foo—bar" on either side of the em dash.

Funny, I'd rather have the break at the start or end of the emdash-implied break than just before or after it, not having to mentally handle some single dangling word divorced from its compatriots.

Ironically, on my phone the only line that ends with an em dash has no spaces in it.

If you want to not have a line break, you shouldn't rely on arbitrary behavior. You should use non-breaking characters like non-breaking spaces and word joiners.

> The reason not to do this is observable in your post on my phone. The spaces cause the word wrapping algorithm to leave a dangling dash at the end of the line which looks ugly. Omitting spaces prevents the word break.

That's an interesting practicality but I don't think it's the cause of the rule: The rule probably long predates automated line breaking. Also, I think automatic line breaking will break compound words at the hyphen; it doesn't require spaces (which is also obvious from a software development point of view: the logic is relatively simple either way):

  Lorem ipsum dolor sit amet, consectetur adipiscing double-
  decker lorem ipsum dolor sit amet, consectetur ...

Preventing the word break doesn't seem very desirable, especially if it causes a large gap.