I have shelves full of books typeset with LaTeX, which by default puts more than a single space at the end of each sentence. It think this looks much better. But, crucially, LaTeX only makes the space fractionally larger. There’s no easy way to do that with most software, and if it’s a choice between one and two spaces, I think two looks odd.
Single spaces introduce a technical problem, which is that the software can’t tell whether a period is at the end of a sentence or merely following an abbreviation. LaTeX’s solution is that you have to manually mark periods that are not sentence ending. People often forget to do this. It also treats runs of multiple spaces as a single space, like Web browsers do.
I stand in the camp of using one space after a period because that’s what I’ve been taught and used growing up.
However, I ran into the problem Michael pointed out just a few days ago. I was doing some translation and the translation management software had issues identifying whether the space after a period marks the end of a sentence or a just an abbreviation. Although this is a very specific scenario that I can manually resolve, it still a legitimate case against single space.
When I get to set style guides, I tend to use the New Hart’s Rules aka Oxford Manual of Style where abbreviations, contractions, and acronyms are used without periods. Dropping periods for abbreviations would enable resolve the issue mentioned above.