Well I never use swiftcast to actually open a fight, that's what precasting is for. But wouldn't it be a DPS loss to sharpcast Fire 1 and then open with F3 instead of just hard casting F3 and following up with Fire 1? The latter is nearly a 50 increase in potency versus sharpcasting F1 > Firestarter. It only takes you about .46 seconds more of total time to get that 50 increase in potency which I find to be worth it. But I guess if you're anticipating casting interruptions, it could be useful.