One thing we have found is that if you want to perform multiple backups 
on
the same box at the same time to reduce the total backup time you must use
polling.  When you use prompted then only one of the schedulers you start
will actually perform the backup.
        A reason to use prompted mode is so you can have tighter control on the
backups.  If you change a backup for any reason and have polling set then
the new information only gets to the node if it polls before its next
scheduled backup.  With prompted the actually schedule doesn't get read
until the server prompts the node to start.  Immediately after this the node
will then ask for its schedule.  Thus the schedule is read just before it is
ran not at polled intervals.

Hope this helps.