i'm a much lower CONJ, but i noticed the same thing. fighting astral mobs, i was throwing down Scourge and then rotating between Aero and Blizzard (for recast times) and getting about 50% more damage total.

ex: scourge hits for ~60, banish hits for ~40. i can rotate those indefinitely or...
scourge hits for ~60, blizzard hits for ~40, aero hits for ~40, at which point scourge has been off recast for a couple of seconds. and two conj spells cost about the same MP as one thm spell!