[Russ]

Look at your spell slots, and how many of them you have... the mage has kick-ass spells, because those slots are all they've got going for them. So a mage can tank, or he can play artillery, or he can play thief (kinda), or he can summon (sorta)... but on an intense basis, he can really only do one of those things really well per day. Forget ToB, forget judgments based on being absolutely high-level, or owning "best items in game..." and look at what they do irrespective: the mage is a specialist, and fighting him on his own terms is a sure loser... get him off his track, and he's fast meat.

The cleric is a generalist. He can turn, heal, buff, summon, blast, or simply kick out Sanctuary and stand in the doorway like the Wall of God(tm) while the bad-guys pathetically try to get at the archers and spell-chuckers in the hallway...