Why, to wipe the hate list. Most people don't think of this, and watch their poor burner or enchanter get hacked to pieces. Cast Screaming Terror on it. You'll mez it for a second, and most likely wipe its agro list. It works well, try it.
Darkness wears off after a set period of time.
Snare overrides Clinging and Engulfing, Dooming and up overrides snare. Root overrides both. Doesn't take long to get the mobs off, just keep that in mind. When its not resisted they don't seem to care. You can get your Cleric out of a mana-low and give your other casters a needed boost when everyone else is full or just alot higher.