I have to vote against this.
on my laptop, shutdown needs to happen _now_. running a disk intensive
operation sporadically at shutdown when my battery is at 10% is a guaranteed
way to trash my filesystems. and leave the job half finished.
a couple of options occur to me:
1. introducing something like anacron to base ( maybe stealing features from
fcron )
2. setting weekly to start at boot with a high nice level
3. adding a FAQ section for workstations/laptops telling people to use
anacron