The real clencher for the pally 4xshammy for me has been fear immunity (or essentially so). Fear really jacks up a boxer and by staggering 4x tremors it's never an issue. The 4x Grounding totems are also insanely powerful. From a DPS perspective, I've gone neck and neck with a mage that had 250 more +dmg than my shaman (maybe the mage sucked iono), and at a measly 700+ dmg I basically 2 shot mobs in heroic mech (easy I know). The pally and resto shammy really make CC unneccesary (it's a PITA anyhow).

In theory the 4x shadow priest + pally would be invincible with hots, vamp, etc.

IMO mixed class teams don't bring enough benefit to the table in order to compensate for the added complexity.