I keep each and every format I come across, even if I can't read it (some lit and pdb files for example). Ok I'm a hoarder. But the reasoning behind my hoarding is this: I don't know what formats I will need in the future, they may be different from the ones I use now - and I'm never sure if something is lost "in translation", I mean in conversion, so having all my options open feels safer. I also have multiple copies in and outside calibre, in external hard drives and usb sticks and whatnot
