Why do you need UI3 ticks at full mana? B3 takes 3.5s to cast, during which time you could be casting other spells that deal more damage. If you had infinite mana you'd never use Blizzard spells for DPS. They're only good to DPS with while your UI3, and Ui3 is only good because it restores your mana, and that's only useful because it allows you to go back to your prime DPS spell, Fire 1.

I understand T3 ticks to trigger TC, but T3 including TC procs is still less dps than F1 w/ firestarter procs under AF3 (*This statement is wrong if the T3 cast off TC doesn't trigger a GCD, if it doesn't then disregard). . There is plenty of time to cast the Thunder spells while you're under the effect of UI3. Also, since the Thunder spells do more DPS than Blizzard spells under UI3, its great to be casting those then instead of spamming Blizzard.