Yeah, keeping the needed packages around would ballon storage size if you want to do it for an entire directory of distro isos, especially if you're interested in historic releases (e.g. for hardware compatibility). I've actually been experimenting with using locally saved packages lately, mostly because HDD+ethernet is about 20 times faster than downloading packages off the internet for me. It's fine as a cache if you're managing several instances of the same distro, but if you want to full-on install several different distros/releases off HDD you'll need to seriously invest in storage space. Might be worth it though ... hmmmmm.