basically it goes down to this

With resto shaman, Your other shamans never stop dpsing, gives You much more survivability and really makes heroic easier, much much easier. My dps in heroics is same with 4 eles, as it is with 3 eles/resto. Four elemental, at some point in the game might be better, when every heroic gets easy.
Also thanks to resto shaman I completed every damn heroic(ocullus not included) still wearing lots of 70 gear, I tried with 4 x ele(which is the set up i used for all of tbc) and well...failed, there was so much healing required that my shammies would go very low on mana, or all they could do is all 4 spam heal me without doing any dps, my resto can just es/rip tide/ spam heal and not go oom...and resto can still dps pretty well, my does around 1400 dps or so.