If you’ve been reading debaday for a while, chances are you now have a heap of packages installed that you’ve tried out, some of which you want to keep installed, and some of which you have forgotten.
Those forgotten packages can cost you. They might contain setuid binaries or running daemons that could compromise your security. Future upgrades take longer, and there will be more to download. Your backups have grown. You are doing backups, right? That’s what duplicity was for.