I have googled a bit on hyphens and dashes after someone corrected my writing and my, there's quite a bit to it, it appears.

It is nice to have all this wealth in typesetting and punctuation, but the question remains: do readers know all this (I didn't), and if not, is the use of all possible dashes with or without space intuitive to the user? The usefulness of punctuation in getting a meaning across is only useful as long as that meaning is understood. As the target audience usually does not consist purely of writers and typesetters, maybe some consideration for the general public is in order.

In my ignorance I agree with the em-dash with spaces, otherwise it joins words together instead of separating them. The only sensible usage of an em-dash or long dash without space I could think of is a thought broken off in the middle of a word, like thi---

But then, ignorance is bliss I suppose. I wouldn't argue with a publisher though, when in Rome etc...