I'm not worried about compression/chd size. It's a matter of trying to document what the real hardware used and not some jackass refurb dude on eBay. It's also a matter of, can the chd be written back onto the correct hard drive type/size to restore a game. Also you don't always know if other things have been user-hacked if you buy a replacement drive, but yeah some might get used in MAME and be marked as bad dump or something and it's better than nothing.