It's true, both the Modern Language Association (MLA) and the Chicago Manual of Style recommend using one space, though neither brands two spaces as wrong, merely out of date. In the days of letterpress and typewriters, which used monospaced fonts, two spaces were considered essential to create a visual break. However, with the advent of computers and proportional fonts this break is apparently unnecessary.
Maybe I'm just a bit claustrophobia, but I like the breathing room and feel that two spaces are still valuable. If I left just one space I'd be afraid that something would emerge from the narrows between sentences, and perhaps make off with my commas. We do one space after each word. Between paragraphs there's the remainder of the line from the paragraph before, and at least a tab mark. Some even use a full blank line in separation. Shouldn't we pay sentences a little more respect?
Sometimes I like a sentence so much that I feel I should leave three spaces after it. I generally resist because then I risk leaving prose convention behind and entering the realm of poetry. But maybe poetry is behind the appeal of leaving two spaces. It is the "white space" that gives extra import to the words on either side. Two spaces offer a compromise between an endless and rough sawtooth of words and uncontainable worship of meaningful statements.