First of all, my name is Dan Gibson, and I'm a recovering two space user. Thanks to Farhad Manjoo at Slate for giving me an extra dose of shame to (hopefully) permanently change my behavior.
What galls me about two-spacers isn't just their numbers. It's their certainty that they're right. Over Thanksgiving dinner last year, I asked people what they considered to be the "correct" number of spaces between sentences. The diners included doctors, computer programmers, and other highly accomplished professionals. Everyone—everyone!—said it was proper to use two spaces. Some people admitted to slipping sometimes and using a single space—but when writing something formal, they were always careful to use two. Others explained they mostly used a single space but felt guilty for violating the two-space "rule." Still others said they used two spaces all the time, and they were thrilled to be so proper. When I pointed out that they were doing it wrong—that, in fact, the correct way to end a sentence is with a period followed by a single, proud, beautiful space—the table balked. "Who says two spaces is wrong?" they wanted to know.
Typographers, that's who. The people who study and design the typewritten word decided long ago that we should use one space, not two, between sentences. That convention was not arrived at casually.