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One thing I've noticed on my PocketBook E-Ink device (~150 ppi), the thin fonts used in most PDF files end up looking dim and gray after anti-aliasing is applied. That's because the strokes used to render the characters are only one or two pixels wide at that resolution. Going up to 300 ppi or more would result in more solid black pixels appearing inside the characters, and thus, better contrast for reading. So, even if you can't see a single pixel clearly, a higher dot pitch would result in higher-contrast rendering of thin fonts when using anti-aliasing. I'm sure it's the same reason printers went well above 300 ppi, as well.
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