What if a package is installed, and puts a script in a run-parts directory or into a .d directory, but isn't configured due to a missing dependancy? The newbie "sysadmin" doesn't know to look for it, and leaves it there, then gets email from cron. Per sends off a tech support question.
This could be prevented by having a place (/etc/cron.scripts, or /etc/cron.d/crontabs) to install things to, then require that the postinst create a symlink during configuration. Hmmm... /etc/crontabs /etc/crontabs/crontab /etc/crontabs/cron.d/ /etc/crontabs/pkg_blah_script ...