I remember back in the days of BC when I asked this – Should I keep VE ( Vampiric Embrace) up? It was like
“Oh noes! You would be a terrible Shadow Priest if you didn’t”
I’ve proved from WWS reports that at the very least now in WOTLK, my Vampiric Embrace, the talent that heals a percentage of a Shadow Priests damage, overheals by 80% on normal fights.
A recent Sapphiron 25 man fight my VE only overhealed by 33% which means it was being useful. The Raid gets a constant AOE, so healing of any kind should be welcome. However I feel I’m being over diligent though in comparison to the other Shadow Priests I run with, but they may have a valid point in not bothering to cast it all the time.
Is the Question as simple as ” Why bother even putting VE up at all?”
Deciding when to put VE up and when not to could become its own theorycraft chapter. I’ve tried to take the safe road. Its part of my normal Spell rotation on boss fights, I have on occasion let it stay dropped off. Eg on Patchwerk I didn’t recast it, but certainly if I am DPSing for every other fight I am keeping VE up.
Its instant cast, it has a decent tick time so you don’t need to reapply often, and its mana free. The only thing your saving by not applying it is the Global cooldown. My personal stance is you should have it up at all times practical.
Yet, If your prepared to put a little bit more thought into it’s use, and the situations you don’t need to have it up for, then you will be saving a global cool down, and that might be important enough to when chasing a dps goal.
So questions to consider before deciding to use/not to Use Vampiric embrace in an encounter.
Who are the Members of your Party & and what damage are they taking?
You and your party are the only ones who will benefit from your Vampiric Embrace. If you have a tank/offtank in your party for most fights I believe there is no good reason for you not to have it up. Same goes for a Healer, if there is unavoidable AOE damage your small trickles of healing may make a difference. The potential benefits far outweigh the very very small difference it will make to your Dps – Keeping tank and healers up are extremely important.
However, on Patchwerk if you have people in your raid that need to stay below a particular amount of health to avoid getting hit by hateful strike then your healing may kill that player. In that particular fight. Don’t.
If your in a Party of pure DPS and or Healers and they are not getting regular damage, ( or only getting damage when they do stupid things) Then your overhealing by 100% and wasting cool downs. You could not put VE up , and no would notice, and probably wouldn’t care. However if you are doing Sarth 3D and people are getting damage from all sorts of things, your Healing could significantly contribute to help keeping people alive. Do it. Stick it on.
In Phase Three of Kel’thuzad when he casts an Iceblock and deals 104% max Hp in damage over 4 Seconds. A tick or two of Vampiric Embrace from one of your spells may also in this case contribute to helping keep that person alive.
When I brought up in guild that the Shadow Priest’s weren’t being as diligent in keeping their VE up, one person in extreme. ( Eg lucky if it gets cast more then once in a full run) the importance of Vampiric Embrace healing was swept aside. ” Well you don’t really need it…… There are other class ability’s that will grant minor heals Eg ” A Pallys Judgement of light procs a healing ability – “granting attacks made against the judged enemy a chance of healing the attacker for 0.2.”* or Even a Shamans healing Totem ect ect..
People just don’t seem interested in Shadow Bandaids. Its a tool however at our disposal, and the health benifits currently outweigh the loss of a global cool down. ( I sound like a Health advert.. )
At best case, between VE and the healing from devouring plague I know I’m getting some form of Health return to me. So untill it starts sigificiantly impacting on my ability to be decently placed in the charts I will keep Vampiric Embrace up all the time. ( Except Patchwerk.. )
Edit: 3.1 Change to Shadow Priests: Vampiric embrace duration increased to 5 min Weeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee Now there is NO excuse not to stick it up and keep it. NO excuse. ( yes I am passionate about this topic.)
Provided by 
Pugnacious Priest's Warcraft Blog 
The spell’s probably not going to make or break a fight, but speaking as a healer, it never hurts to have more healing out there. It just makes the fight that much easier. Also, healers hate having to heal a tiny amount of damage, so if you can have something to tick that away, hell’s bells, why not? Trash mobs, whatever, but I got ❤ for the SPriest who keeps it ticking on a boss.
I pretty much run with the rule of not on trash but yes on boss.
It costs 0 manna now and is an instant cast, we dont have to worry about threat any more so its definitly a help and in my eyes surely anything that can help should be used.
For me personally, I keep VE up mainly for myself. As a rule of thumb, if the fight has an AoE component that requires an extensive healing, I will keep it up. Otherwise, I will keep it up only if I can spare some GCD. It’s not so much for the party healing, but I figure, the healers would appreciate having one less person to heal (or at least not so much).
Whether it’s a habit or not, I keep VE up almost always for boss fights and trash mobs with 4+ mobs. I’ve only had the experience of fighting Patchwerk once with my spriest, but obviously I didn’t keep it up then.