In my opinion, I probably would not go with the shaman. I don't believe at 70 a shaman can be a primary healer. I could be wrong, but you have already stated that you have a priest ready to substitute so thats not an issue. You have the tank and healer coverered, quite well actually with your tank being a paladin.

Now the question regarding DPS. I'm a firm believer in simplicity. Filling your three dps slots with classes that have generally the same skills is quite abit easier then trying to manage a fight with many different timings. What i mean by this is, 3x times warlock, mage, hunter, even shaman will work well if they are on the same spec and all their casting is in sync. Setting up a warlock, hunter and shaman as your dpsers can be tough as they all have different timers for their spells and attacks.

I don't know the casting times on the shaman's shocks, but if they have the same casting time as say a mage's frostbolt, or scorch or even a warlocks shadowbolt. You could set up your controls and class skills to take full advantage of that. This will give you more control over exactly who you burn down and when you do it. An example of not have 3x a class is what I did with my 3 dpsers. I added a warlock later on after i had gotten to 70. I speced that warlock to destruction because it worked really well with the mage's frostbolts. Things like Firstblast and Shadowburn could be casted at the same time. Scorch and Searing pain worked well together. etc.. etc..

Keep in mind, this timing issue is really only relevent if you are using one input to control your dps. As a 5-boxer who tries to set up the "perfect" group you are going to work more for control and finesse. Where a 5-boxer of all the same class is about brute force and blowing their enemies up.

Corpse runs are not an issue with 3 warlocks. The soulstone cools down every 30 minutes. So you will be able to rez your priest every 10 minutes mid battle or after a wipe. After every wipe you will spend 2-3 minutes eating/drink and rebuffing anyways. The soulstone must be put on the priest before the priest or the warlock dies. But, it can be cast in combat as an oh crap gonna wipe last resort.

I'm sorry if I didn't really answer your question. I just don't know enough about shamans and why it would be worth having him. Regarding the chain mail, it is quite frustrating having so many chain and leather pieces not used. But having a high level enchanter in your set up really helps with gathering disenchants of all the bop items you won't be using.