For armies without a full t4+ tank having only 175k hps, when MCP or Warlord triples (or quads) at 80k - you are dead before anyone can heal you.
Although, I haven't used a cleric (ever) and don't know their healing capabilities. Seems to me that 2 4.0 PALs with level 20 oracle can heal 32k per swing (combined) and do so with double/triple attacks, etc and that feels more efficient than a single toon casting one spell. Then again - i am talking out my arse 'cause I don't know what a cleric can do.
Clerics cast divine intervention which allows the tank to live rather than be killed and it is on a 1.5 minute cooldown. Also clerics 4.0 procs heals that heal for as much as a pallies. Also im sure you've noticed times where the tank just sits at 30% or what ever for a few rounds and just doesn't get picked up by procs. That is where clerics instant cast gheal comes into play. I just recently killed my first warlord w/ a pally/cleric/mage/necro/monk/shaman and my pally was almost full t3 missing helm/boots as the MT because perma stonewall is stupid good.
On a side note procs only happen once per round so it doesn't matter if something doubles or singles, but pallies do get AA to make them proc more.
small edit about clerics: starting at cleric 3.5 they get a targeted 30 second w/ 1 minute cooldown stonewall.
