I was told a few years ago at a seminar that our habit of double spacing after a sentence was a carry-over from the manuscript/codex days, which, in my experience, is exactly the opposite of the truth. Not only were these ancient texts not double-spaced between sentences, they used NO spaces (and often no punctuation) between sentences. So this may simply be an evolving convention with no rational source, driven by the same fewer-keystrokes-is-better mentality as "Im 1 and ur 1 2" [period intentionally missing]

"I don't know which is worse: ignorance or apathy. And, frankly, I don't care." - Anonymous