If you notice this was in reply to someone else showing their rotation and asking what I think. That's not my rotation. My rotation is:

Swiftcast > Thunder 3 > Fire 3 > Fire til OOM > Blizzard 3 > Thunder 2 > repeat. Using thundercloud for Thunder 3 and firestarter procs at end of casts or when I need to move or saving to get back into astral from umbral after regen.

Read Thunder 2/3. I get full mana by the time I finish Thunder 2, so why would I cast Thunder 3? Of course you can get back to full MP after casting 3, it's .5 seconds longer without buffs. Look at the benefits of both, 3 = 3.5 cast, 60 initial potency, same dot damage, and +3 second duration on dot. I use thundercloud procs on 3, otherwise I'm casting 2 since I regen to full MP by the time my Fire 3 goes off.