People mostly save it because to use it you have to pause after the last cast and wait until your Fire 1 actually lands on target and applies Firestarter on you (or may not) so you will be losing extra ~1 second every Blizzard III just to understand if Firestarter procs. So generally you will just chaincast Blizzard III after the last Fire 1 and you will understand that Firestarter procced in the middle of the cast. So I am not sure if it is worth it to make that pause.
