5 shamans in WoTLK heroics?

[emesis]

More than 2 shamans don't stack in any meaningful way. Add a warlock or moonkin and you just got a flat 12% spell damage increase across the board. Add a third shaman and you get maybe some more mana regen. You only have a couple fire totems that add spell damage, and only 1 heroism every 5 minutes.

So why do two shamans stack then? This makes no sense.

"Maybe some more mana regen" ... How about

1. Multiple healing stream totems making splash damage largely ignorable in many encounters
2. Multiple chain heals making the occasional AoE heal very simple to do without a dedicated AoE healer
3. Staggered multiple thunderstorms is very nice on a PvP server. Can also be used in instances
4. Multiple wipe recoveries with reincarnation
5. Linearly scaling, strong AoE damage without threat in 3.0.8 with the magma totem buffs

To be sure, there are any number of viable 5-man boxing combinations, but IMO multiple shamans are still a great combination. They are very easy to manage as well.

An unholy DK also brings a 12% damage bonus to the table (Ebon Plaguebringer) - in fact a much nicer once (works on multiple mobs, as opposed to Earth and Moon or Curse of Elements, which are single target). I do agree that bringing one of these buffs to the table is the way to go. In my case, I'm bringing up an unholy DK to tank for my shamans. I have a druid in the group as well as a mage and a pally, however, I'm probably going to go with DK/4 shammies as my main group. With the change to elemental oath (now includes 10% bonus damage for the proccing shammie when clearcasting), I need to keep 2/2 elemental oath on all the shammies, so Moonkin Aura no longer would be an additional benefit.