I worked with someone who used two spaces after a period, and it made it extremely difficult to proof-read and make corrections on her papers, as you have to go through the entire thing and check for spacing consistency. There's no need for two spaces after a period; if you can't figure out where a sentence ends, that's your problem, not mine.
Find and replace. Takes a few seconds to do. I work in many groups that write together and a find and replace for consistancy is always the last thing I do.
Two spaces looks better in my opinion, but it appears that I'm in a minority here. I'm fairly certain typesetters do it for monetary reasons. A few extra spaces might lead to a few more pages in a manuscript, which is a few more dollars to pay for materials. Now that lots of text is digital, we should be free to do as we wish.
Personally, I'm a double space between words within a sentence nazi though. Those unecessary extra spaces between words really look bad (especially when using justified text so the double spaces between words can be an inch long in some cases.)