I currently have a level 46 Holy priest who I am taking off the shelf of alts and running again. I am duoing with a same level Enhancement Shammy. I (painfully) leveled this toon to 45 as Holy a long time ago and then made her a bank alt.
I plan on having her play Holy/Disc when I hit Outlands. But I have 12 levels to go before that happens. For those 12 levels I just need her to keep the shammy alive and kicking while doing a little dps. But I also need to heal instances when necessary.
I main healed for a group in Uldaman last night and used Renew for 90% of my healing. I would throw in the odd Heal and Flash Heal every now and then. I was a little higher level than the instance needed and had my healing set on. I found that I didn't use my Greater Heal even once, I just didn't need it. In fact I was overhealing when I threw my regular Heal and Flash spells. This made me wonder if a full Holy priest is a little overkill for low to mid level instances.
I know that a shadow priest can main heal instances all the way to Outland. Would it be worth it to switch to Shadow for the next 12 levels for the extra dps? I like playing Holy and have no issues with sticking with that. I just want to be as useful as possible while leveling.