I've never needed a resto spec shaman. I use 5x shaman and all of them have been spec'd elemental through level 70. Pre-BC instances I never ran into a problem and could manage pulls with stoneclaw totems (on round-robin) and grounding totems. Only tough instance that I recall was that section of sunken temple where the dragons are nature spell immune.

BC instances I learned 1. I needed to wear a shield, 2. Needed a /stopcasting macro /cast lesser healing wave macro to have all 5 characters heal a specific target when needed, and 3. Earth elemental totems made most boss fights easy as cake.

FYI 5 elemental shamans can do heroics. I just spec'ed out of my PVP spec of 40/21 (Natures Swiftness) and back into 41/20 (Totem of Wrath).

I have heard it's easier to level up shamans as enhancement.