It's what I'm currently doing, and each spec is so situationally powerful, I now feel obligated to bring both to Ulduar, so dual-spec really didn't allow me to easily have a pvp or alternate role (shadow) spec at all. My PvE role is healer, and to be the best possible healer I feel the need to bring disc and holy. WTB Quad-spec?
I myself took the disc/holy dual spec. I don't have the shadow gear and i am the guilds main healer so i have come equipped with both specs and gear as needed. Unless you are just a part time healer or you have another active healer in your guild then you can go shadow as one spec. I would love to go shadow again but i'm not willing to lose my raid spot over it.