You can't tell cron "when this happens, run this job." For instance, if you want to be notified when you start to run low on disk space, or if the load on a machine is above a certain threshold. You could write scripts that check those things, and then run them frequently. But it'd be better if you could just count on the scheduler to do that for you.
There's also the less than friendly formatting to cron. When I've worked in hosting, I found plenty of cron jobs that were either running too often because the owner mis-formatted the time fields, or that didn't run as often as expected for the same reason. Back up jobs that should have been running daily were running once a month. Oops.