It's because if we don't have it, you go squish. And no shieldwall will help you
Well equipped Resto Shammies rarely go OOM. More of an issue in Arena, though, as less gear is oriented towards MP5.
For a Resto Sham, the min requirements I'd be looking for would be 1500 +healing and an MP5 (without Water Shield) of around 120, unbuffed. I have seen Resto's with less, and in easier Heroics, 13-1400 healing won't be a huge issue if the player is quick and savvy, but the MP5 issue is huge - as an insanely equipped Raid healer, my buffed MP5 is over 300 while casting - meaning that clever use of Chain Heal can see me lose virtually NO mana.
Interestingly, it is worth having 2, even 3 ranks of CH on your toolbar, as they really do provide excellent healing efficiency - and unless you are really having to heal hard, you should not be using Rank 5 CH, but rather Rank 4 - healing co-efficient is the same as the higher rank, and the effect is in the same range, for less mana.
Pewpew-wise, a resto shuold be aware that they are there mainly to honk the opponent off with interrupts and buff removal. You can stack Elemental Talents along with your resto tree, but tbh, you would be smarter using the mana boost in enhance, and getting instant cast Ghost Wolf - that ability is very handy, and also just plain "cool" to use!
