I am by no means as experienced as you guys. But I started with a prot pally and 4 shaman.

Now I have only one shaman left. First I added a Frostmage for 1 Ele Shammy, and I love it. Not only the int buff is nice, but Frostbolt + Blizzard are badass DPS - plus the frosteffects you get, plus the 2% caster crit on each frosthit (stacks up to 5 times). I do not even want to mention PORTALS *yikes*.
Then I changed my resto shaman vs a disc priest and the priest heals so much better. Shield, Groupheal, Sta buff, WP buff, Fear immunity for tank etc. add so much more than my resto could. Yesterday I changed another Ele shamy for a Moonkin and again, I love it. Moonkin caster buffs, Mana regen buff, Moonkin ultimates, Moonkin AoE, 13% caster dmg increase on mobs, to-hit-debuff on bosses etc. etc. All this stacks up.

Compared together, my mixed group DPS is 35% higher (!) than my 1 Pala + 4 Shamy DPS. And its so much more enjoyable to have different chars to equip, explore their strenghts and weaknesses ... and have a whole bunch of toons available for your solo-play whenever you feel to.

I will never go back to shamans, I have one Ele shamy left for ingrim and caster-speed totems ... and yes, the mana-reg totems are nice, but so what. Just my opinion.