You should have been here when the first Crystal towers came around and they required the 4 FATES be completed to gain access. That was a lot of fun when the entire server was out there competing for the same fates @_@.

They touch on this a bit with concerns about the relic quest (also requires fates on a certain stage).

Dev response is that they are timer based, but only in relation to the completion of other fates in the area. In short, if you want your fate to appear, it will happen faster if you complete other fates also in the zone.