I've always been a single spacer. I understood the purpose in monospace, I otherwise never understood the practice. Almost everyone I've heard only practices it because that's how they learned. That reason wasn't good enough, especially when that was (mostly) rooted in the typewriter, and we're using the modern day computer. Some (justified) reasons are enlightening.

I was always under the impression that in a proper typeset manuscript, sentence spacing should be slightly wider than a standard single space. Typographers make this change when they typeset right? Isn't there an alternate ASCII character for this purpose (entirely not sure, I might be thinking of special characters in certain fonts)