I liked it when i did this with a pally buddy of mine (my first Dons were my spriests on Turalyon). Problem is that as soon as things get ugly, you end up having to shift out of shadowform to kick off a real heal every so often. The burst DPS is nice here and there but overall there are other combinations that are much more effective. To answer a specific question, no, 4x VE+VT are NOT as effective as 3 DPSing + 1 real healer. Once you get enough +healing, 6k heals are regular. Takes a LOT of VE to heal for 6k. Now, an interesting sidebar to this is holy AoE priests. That is interesting. Check out the talent called Surge of Light. Cast holy nova a couple of times in an AoE situation and almost all of them have surge of light up, which means free insta-smite. :P A niche group to be sure but fairly interesting coupled with a pally's judgements.