I use Calibre to convert all of my ebooks before copying to my Kindle, and ensure that they have ragged right edges (aka left justification).

But sometimes, I'll download a sample to my Kindle, and read it immediately... and if it's fully justified, it tends to look like garbage. Without hyphenation, there are monstrously large spaces between lots of words. It's just terrible.

I don't expect LaTeX-quality output, since even LaTeX requires some hand-holding sometimes, but let's be honest: the Kindle's typesetting is trash. Ragged right makes it pretty readable, and I'm glad I can use Calibre to force it, but if I couldn't? I'm not sure what I'd do.