Mini-global-cool-down. Try casting Blast Wave and follow with a fireball AT THE SAME TIME. It is not on a regular instant-cast cooldown, and does not take as long to recover from casting as spells like Living Bomb and Flame Orb. It's damage is low(ish) but is should not^ slow down your regular rotation one bit when cast in-between spells. Therefore it should not be looked at the same as other instant-cast spell, and not be compared as DPS to any other spell without considering fact that it can be cast without slowing a regular rotation down.
Try simultaneously casting another spell following a Blast Wave as fast as you can. You will be lucky to beat the speed of it's GDC. It will not interrupt a fireball spam for more than a split second, and will chance crits and procs like any spell. 2 targets you may want to use simply to chance impact. 3+ targets it simply cannot be denied.