And now, the reasoning behind the line breaks in Markdown, from the horse's mouth:

The implication of the "one or more consecutive lines of text" rule is that Markdown supports "hard-wrapped" text paragraphs. This differs significantly from most other text-to-HTML formatters (including Movable Type's "Convert Line Breaks" option) which translate every line break character in a paragraph into a <br /> tag.

When you do want to insert a <br /> break tag using Markdown, you end a line with two or more spaces, then type return.

Yes, this takes a tad more effort to create a <br />, but a simplistic "every line break is a
" rule wouldn't work for Markdown. Markdown's email-style blockquoting and multi-paragraph list items work best — and look better — when you format them with hard breaks.

I feel kinda like I'm arguing for JSHint over JSLint at this point (I WANT OPTIONS!), but this isn't code, it's composing content. (It's a distinction for me, anyhow. ;^D)

Wonder what the "hard-wrapped" use case is? Pastes from emails? Hard wraps in BBEdit? (That second one seems unlikely.) Editing content that already contains hard wraps always stinks. I can't imagine Gruber's doing that.