I don't use the update anymore. Many books are still unread in my account so its not about re-reading. I too buy lots of back list stuff and also lots of sales items.
I used to have the auto update turned on. Now I have devices that I might not use, but they are all still registered to my account back to my first kindle the K1. The reason I don't update anymore is that I got screwed over by authors and that turned me off from doing it. I only know these cases because of other readers and I came across it. Who knows what other books got messed with.
One case it was a book, out of print re-released after like 20 years. I so wanted that book to read again. So I bought it outright. I didn't get to reading it right away and I had the auto update on. Next thing I knew readers complained that the author "cleansed" the book. Made it PC basically for today's standards. Basically mutilated the book that was so beloved by many. Unfortunately, mine had already been replaced with the new version.
This is unfortunately not a isolated thing with authors that wrote certain books 20-30 years ago, especially in the historical romance genre.
The other incident known to me was a book I bought, didn't get to it right away and some time after the author replaced that one with 25% of the book. Basically they decided to chop the book up in pieces and sell it as a serial.
I did get my money back on that one and I hope amazon keeps track of that better, but I guess how would they know, unless someone reads the thing.
So that is why I do not update anything anymore. At least not before I read. Now say I read a book and I find out from the start that there are lots of formatting errors or spell, I can always check in the account if there is an update at that time. At least at this point and couple of years back now, I expect the books to be published in the best condition as possible. Back when I bought paperbacks, I couldn't go to the store and turn it in for another print either. So shouldn't be an issue. So far it hasn't been really.
And also, since they took away the drop down where you could see all books that had an update, its impossible to find anything if you have a good size library. And the hated infinity scroll makes it even worse. I hate that thing. I'd sit there for hours trying to scroll, scroll scroll.