Because people working with timestamps like to write code that says things like:
// schedule another run for tomorrow
schedule_event(now() + 86400)
...which doesn't actually work when leap-seconds are involved (things start to drift by a second). If you specify that leap seconds get replayed, it works.
Strange then that we'd scold a developer for assuming the number of hours in a day, or the timezone, or whatever, but not for using Unix time as an absolute time, when in fact it is not an absolute time at all and even caters to the same error in thought.
