In my view, the problem here is in the changing definition of "space". The classic rule is ONE space after a period before the next sentence. That works perfectly in the era of monospaced typewriters with pica (10 characters per inch) or elite (12) letter positions. But today almost all work is being done on word processing equipment that uses proportional spacing fonts - ones in which the width of a letter depends on the letter itself. In such fonts, a "space" is a narrow letter and gets, actually, LESS width than the "average" letter like "a" or "k". Its width is more like an "i" or "l". Hence, one such "space" actually looks not quite enough spacing between sentences. I believe, like many, that TWO such "spaces" makes the layout clearer and easier to read. So in today's world I use two spaces after a period. To me, that is the right way to accomplish what was the aim of the old original rule - clarity of the written document.