There will be one acceptable spell rotation in PvE: Arcane Blast x 4 Arcane Missiles (if Missile Barrage has procced). If Missile Barrage fails to proc after four consecutive 40% chances for it to do so (and we all know this will happen frequently, thank you random number generation), you will keep casting fully stacked Arcane Blasts until it does, since the mana cost is not prohibitive to use aion gold.
That is twice the chance, according to my math, or to be more scientific, casting other spells will still grant you the 20% proc chance. Which of course begs the question: why would you cast anything else? You would not, that is the answer. Also, why would you ever, ever cast a non-Missile Barraged Arcane Missiles? Let us weigh the options here:
Arcane Barrage will have no further use in PvE, except as a way to continue to DPS on the move. This saddens me, since it is a 51 point talent, and it is an awesome spell, and I love it dearly. I pray they can find a way to make it PvE viable again, but doing so without upsetting the PvP applecart is probably going to be difficult.