I tend to run a team until I hit an obstacle then adjust. I've run ALOT of compositions, the only compositions i've run at level cap are:

Prot Paladin, Holy Priest, 3x Mage
Prot Paladin, Holy Priest, 2x Mage, Hunter
Prot Paladin, Holy Priest, 3x Shaman
Prot Paladin, Resto Shaman, 3x Shaman
5x Shaman
Prot Paladin, 4x Shaman

I am currently working on 4 Druids, a DK, and I am considering a Warlock to experiment with... My motivation to look into these compostions is the added synergy.

I think Shaman stacking (or Druid stacking) is powerful primarily due to how robust it is; not how simple it is..

An Elemenatal Shaman, or a Boomkin are equivalent to 1 DPS, and about 0.33 of a healer. (ok, this is debateable.... ts definately higher than 0.25 but lower than 0.5)

Running with four of them means you can have dps bursts equal to 4 dps, and healing bursts equal to 1.33 heals. If the fight is light on heals, you can dps it down all the faster, if its heavy on the heals, you can react achieve the required healing amount all the easier. But more importantly, if you lose a random team member, it is still much easier to recover. Losing a shaman drops you to 3 dps burst or 1 healer burst... In a specialized team your either going to be at 0.66 dps sustained, or 0 healing... (assuming no shamans no offhealer dpses)

Admittingly, 2 Elemenatl Shaman, and 2 Boomkins would have this same level of robustness with more synergy. But would be differentiated from a pure stack by the distinctions identified elsewhere in this thread (setup complexity, loot distribution)

In my case tho, I don't think my 4 shaman team has been more successful for me because it was simpler to setup, or because they have a dps advantage over any other class, but because they are just more robust.