I am a little surprised that Farhad Manjoo missed out on one important reason for not using two spaces after each sentence:
But I actually think aesthetics are the best argument in favor of one space over two. One space is simpler, cleaner, and more visually pleasing (it also requires less work, which isn’t nothing). A page of text with two spaces between every sentence looks riddled with holes; a page of text with an ordinary space looks just as it should.
A simpler reason is that except for documents used at work, almost all of what you write will end up as text in a browser – Email, Facebook, Twitter, blogs, comments, etc. And text in a browser follows HTML rules which is that multiple spaces are treated as a single space unless you explicitly marked them as non-breaking space. And who has time for that?
Another thing that is surely going out is the underline. When writing on paper, we mark important text in underline so that it stands out. Doing the same thing on the web makes the same text seem like a hyperlink inviting a click. Hyperlinked text is usually in a different color, but because some websites overrule that, any underlined text, regardless of whether it is the same or different color, looks like a hyperlink. So for marking something as important, you have to resort to bold, italics, or increasing the font size.
Google Docs has already started the process where documents and spreadsheets are viewed online and thereby follows the standard conventions of the Web. I think it is safe to say that corporations (and your office) will eventually get there. So the sooner we stop being a stickler for old grammar rules and learn how to optimize our writing for online viewing, the better.