You are still leveling and probably dont have alot of mana regen. Starting off once you get to 80 and are doing heroics, having a dedicated healer is nice. Once you start to gear up a bit then you barely get hit as it is as a pally or warrior.
Having 4 dps then allows you to clear instances faster. I have 3 macros for healing when I run my 4 elementals. 1 button makes 2 of my shaman heal tank, 1 button makes the other 2 heal tank and 1 button makes all 4 heal tank. My last button is rarely used. 2 of my shamans is usually enough to heal my pally out of trouble. He can then go back to relying on healing totems.