Contary to popular belief, a multiclassed Cleric/Mage is not worse off because of the lower levels in either class. Assuming it is 75% efficiency on either side, that is a total of 150% efficiency.
A Cleric/Mage is truly a power to behold, and can easily kick any Mage or Cleric in a fair fight; because you see... you can combine the cheese potential of both classes and mix them around to make new cheese.
Time Stop + Harm is a classic cheese from the Cleric/Mage, and if you think spamming spells as a Mage under Improved Alacrity is impressive, wait till you witness what a Cleric/Mage who has double the payload can do.
Just my $0.02 worth on the powergaming aspect of Aerie.