IIRC Replenishment doesn't stack, the newer one will overwrite the older one.

Shamans do have some "can't stop, mobs will eat me" mojo going on with Earth Shield, Riptide, Healing Stream, Nature's Swiftness when needed, and eventually Spiritwalker's Grace and Unleash Elements. And while not instant cast, Healing Surge is a really nice heal with a low cast time.

But, that's just my opinion playing with a shammy, I'm sure someone else can come along and give some pro-druid rebuttal.