Well if we are delayed by transportation, illness etc, they better seat us after our ADR time or waive the fee.
They have a policy that they will seat you 15 minutes after your reservation time. For most places, your reservation will then be auto cancelled. WDW also recommends you give yourself 1 hr using WDW bus transportation to get to a dining reservation.
If something comes up...call. Don't simply show up late and demand they seat you regardless. I have found WDW to be most accommodating when you give them as much advance notification as possible.
They even instituted another number simply for cancellations.