(as a 19 lock) I'd prefer Destro over demo in pugs, I liked it to cast a big cb, but the idea of saving shards as demo and use them all at once will do big pressure as well. In Duell I use Affliction, the drain life is way stronger and I don't have to cast that much, I am able to get f2p priests oom or to kill  rogues. I'd say, all specs have their usage and are different.
				
			 
				 
 
		 
 
		