I can always retypeset the text... but I'm not a professional editor/typesetter. You often lose parts/phrases/words the author wanted emphasized with italic and bold. Blockquotes can be gone. Even paragraph indentations in the worst offenders. I couldn't recreate that if I tried. Lord forbid there's a list/table/figure (even in some of the fiction I've read, they'll have those... weirdo science fiction novels, after all). I've gotten pretty good at fixing epubs with Calibre's editor, some of these are salvageable. Just finished with one where they split the chapters wrong (not at the chapter headings, but in between for some reason). And I'll often go get a high-res cover image off the publisher's website; they like to use bad sized-for-favicon scans off a random google image search for some reason.

But for me, the bad OCR ebooks can be painful to read.