I pretty much use it only to convert Amazon books to epubs. Occasionally I will use it to check the cover images on a batch of epubs. But if I find I want to change a cover, it's quicker to just unzip the epub and replace the cover image than to do an epub to epub conversion in Calibre.
I don't keep any books in the Calibre library--I move them into my own backup file and keep track of them on a spreadsheet.
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