[gmw]

There are lots of heated arguments on the 'net, with various people arguing that it's a hang-over from fixed-width fonts on typewriters - but extra spacing after the full stop has a much longer history than that. (I had a really good link somewhere that went into some of the history in detail, if I find it again I'll post it.) The typewriter aspect came about only because two spaces were the only way to simulate the traditional larger space.

BUT, whatever the history, there are issues with double-spaces now. Firstly, ebooks are all HTML anyway, so your extra space is ignored. (So in this case it doesn't matter whether you have them or not.) But, more significantly, when producing print editions, and since we no longer do the typesetting manually, if you combine double-spacing after the full stop with full justification the effect is often not pretty (in my experiments anyway - obviously the end result is dependent also on things like your hyphenation settings).

I still write using two spaces after the full stop - it's just sort of built in, and most of the time I prefer my text that way, so I haven't bothered to try and change my habits. BUT when preparing to publish I do a change-all to remove them. (Because all my text uses styles with no extra spacing anywhere, I find that I can just do a change-all from (two spaces) to (one space) repeatedly until none are found - which is neat because it picks up all those accidental ones as well.)