The fewer classes you have, the more efficient your set up will be.

With just one class, you could have every ability available for casting.
With five different classes, you probably have to choose the most useful 4-5 abilities per toon.
Granted Paladin/Shaman is two classes and not five, but it still forces you to discard less used abilities.

The paladin is currently the best AoE tank. Prior to WotLK, the Pally is the multi-boxer tank of choice.
In WotLK, every tank will be able to hit everything in melee range, with their AoE moves: Thunderclap, Whirlwind, Swipe, etc.
So, for the future, other tanks might become very good options too.

In general, play a melee toon (tank) and hotkey the ranged toons works well.
A tank can survive numerous pulls where the squishies would not.

On the flip side, a 5th shaman gives you an extra four totems and a lot more firepower.
As a 62, without pre-BC raid gear, Ramparts won't be easy without a tank.
With a lot of crafted/raid/badge/arena gear on every toon at 70, you might outgear most heroics and have an easier time with them.
The concern should be your relative strength while leveling or gearing up.