I have 4 shammys at 80 and I have never cast hex on any of them, not even to try it. As Lyonheart says, CC isnt needed these days as all the instances are a joke. Also your healer shouldnt be dying because he shouldnt be getting hit, thats what the tank is for after all. A lot of people play with 4 ele shammys and 1 pala tank. Personally I use 3 dps, 1 tank and 1 healer. Its all a matter of playstyle preference.

I dont know what level you are at now, but I am pretty sure heroics are going to be a massive problem for you without a healer when you first start. Most of the trash packs should be ok, but I doubt you will be able to tank any bosses with no healer. Mages can put out more dps than shammys in top end gear, but we are talking about gear that is only obtainable from doing high end raids.

My advice would be to drop 1 mage for a shaman and dual spec him resto/ele. Then you have the best of both worlds. The shaman will boost your mages dps considerably with totems and heroism and can heal when needed also. If you are still around sfk level which I guess you are as that is an instance you mentioned, then it wouldnt take long at all to level a shammy solo to catch him up with the others.