Casting other spells will still grant you the 20% proc chance. Which of course begs the question: why would you cast anything else? You must know cheap wow gold, that is the answer. Here are the implications for Arcane Mages should these changes make it through the testing process and then not get nerfed 36 hours after the patch drops.
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. 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.