When I was at my first design/publishing job I looked into this issue seriously, and learned that it's obsolete / holdover from the old times. Today's fonts have actual kerning that takes care of things like this, and double spacing looks just wrong.
*edit* as that blurb said - if you need any proof for this, just look at about *anything* published nowadays (and, probably 20 years back or more). You'll never find double spacing after any punctuation.