However, I contend that you cannot say any longer that a Cleric is absolutely necessary to have any longer. the new 3E Rules have introduced Bardic healing, Faster hit point recovery, more easily attainable magic items, more flexible paladin healing, etc. - all to remove the absolute necessity that clerics had under earlier editions.
I played in a group once that had THIRTEEN players in it. The party consisted of fighters, rangers, paladins, rogues, monks, wizards, bards and sorcerers - and NOT ONE CLERIC, until we hit 9th level.
How did we do it?
Wands and potions - LOTS of them. We fought ungodly HORDES of orcs, lizard men, frog people, multi-classed kobold tribes, beasts magical and non-magical, etc. - And still we came through time and again.
It is indeed possible to adventure without a cleric in the party. NOT AS EASY, but still absolutely possible.