Not quite true. For example, there are two enemy next to each other and I kill one of them with a concentration spell. The kill-cam raises its ugly head, depletes my magicka, and now my spell is useless against the second enemy. Happens frequently.
Actually what I said WAS true, you just cut out the second part, about killing one mob before the others have agro yet. But unless you're killing something with only 8 or so hit points, the other mobs WILL agro before the first one is dead from the concentration attack.