I am currently 10 boxing and I would have to agree with TheRadin that priest healers are very powerful. They have PoM and CoH for fire and forget group healing. Also if your raid is all casters then they will most likely all be piled up on each other which makes holy nova x2 very powerful.

Another reason to go with matching tanks and healers is from a management perspective. When you jump to 10 boxing the amount of time you spend researching specs / gear and setting characters up is incredible. LOL looking back is sort of wish that I went with 2 pallys, 2 priests, and six shammys just so it didn't take hours making changes to my team after every patch.