Actually if you pay attention it 'stretches' out the cast timer... Basically it is faster until the damage hits then suddenly it takes longer. I've been doing a lot of testing with this, but the numbers still support going Fire 3 >>> Fire until Firestarter >>> Instacast Fire 3.
i agree, once we get that talent, this trick takes a backseat. BUT its still a handy trick if you need mana, cast Blizz III, then get 2 faster then normal fire III's then into Fire I until your mana is dry again