There is often a need to do some post copy/sync operation when copy/sync task did (copied) something. So one parameter with property to set in Sync/Copy task would be very helpfull. It eliminates to add special uptodate before copy it check manualy if something is to be done. In case of Sync I have no idea how to write such detection. So puting this functionality inside Sync/Copy is the most comfortable.
There was already a similar discussion on a list, but "on-update" property proposal have a flaw - a previous build could execute <sync> and then fail before the result is checked and the modification will be forgotten. Without bug 17973 (<final> task) that could persist the properties, "on-update" property will not make a difference. On a side note I would rather have a property on a target than on on a task, but I see when both could be useful.