Even for scheduling systems, some historical time zone data may still be needed. Consider if you have a recurring task in a time zone that has an upcoming standard offset change or DST rule change. Before the change, you only needed to know that it was coming up. But as soon as the change has come into effect, the old rules are now the "historical data". Sure - you might* not need the old rules for scheduling the next occurrence, but your application may have other reasons to reference the old rules (reports, etc.)
* I've also worked on some scheduling systems where the original first occurrence date is kept in terms of local time, and future dates are based on that original date. In this case, the old and the new rules must both be available to schedule any future occurrence.