It is not just to fit the screen, it also fits our line orientated version control better.

I don't know if this is suitable for large works(books), but for technical documentation I have my plain text source with one line per sentence, actually I go further than than and usually have one line per punctuation. The raw source reads a little hard but the version control diffs are much cleaner and editing is is easier. Most formats(html, troff, tex) ignore manual line returns anyway.