User blog comment:A.J AL/Season six plot hole./@comment-5619706-20121215033415/@comment-5115351-20121216024824

Imperiex, it is stated that the angels dont use the souls, not that they cant. This is Samandriel's quote from Whats up tiger mommy:"We guard the souls in Heaven. We don't horse-trade them."

But then again, Castiel was pretty desperate. He was willing to do anything to stop Raphael. Going as far as saving the titanic to produce more souls for his "war machine".

As for why Raphael did not attemp to absorb heavens souls, you're right, he probably didnt feel he needed to do such a thing at the time. Though at the very end he did express interest in becoming the new god. So who knows. He might have gone back to heaven and absorbed its souls had Castiel not killed him.