[Gremlin]

I can see the logic behind copying each file to another location if you consider "Calibre" to be converting software. Nobody usually read more than several books at a time on their device, so the space wastage disadvantages are greatly overcome by keeping all versions in order.

I can't see the logic behind doubling the disk space if you consider "Calibre" to be catalog software. Catalogs are called (by definition) for dealing with huge amounts of staff. When remembering what each book that you have contains, its table of contents, where have you placed it (each book can have multiple tags, while multiple folders/shortcuts are not feasible) is generally out of reach. Doubling several gigs is quite costly in terms of disk space and generally should be avoided without an extremely good reason.