They're wrong. *Typing* two spaces should be the rule. Sentence boundaries are incredibly useful semantic information that should be preserved in the underlying data even if it's not being displayed to humans. *Displaying* two spaces is the problem. The problem is software that displays two spaces when you type two. Get better software. - Amit Patel

*continues doing it* - Big Joe Silenced

Typing two spaces seems like a very inefficient means of accomplishing the semantic markup you're lookig for, Amit. It's difficult to tell if you've actually done it, leading to dirty data. It's certainly not universal, and likely to become less-so, and algorithmic solutions for distinguishing mid-sentence punctuation from sentence ends seems to be getting better, and there's a continual incentive for improvement due to auto-capitalization on mobile platforms. The number of times it's important to me-the-author to give the computer a specific sentence divider more specific than a period is vanishingly small, and it seems uneven to make sure every platform displays only one space instead of two, when almost no platform recognizes two spaces as semantically significant. - Kevin Fox

It's much easier to read text when the spacing between sentences is larger than the spacing between words. This is the default in TeX. - David desJardins