Relmstein had an interesting series recently on why healing classes are so unpopular. I know from experience that healing classes are rare, because some days there's a lengthy queue for my time starting from the instant I log on. The following is representative of my daily login attempt -- and is usually enough to make me go log on my rogue!
Welcome to World of Warcraft!
[Guild] Friendlymage: Hey Lizzy - want to heal Scholomance?
[Randomshaman] whispers: need healer for BRD! last spot!!
[Randomhunter] whispers: LBRS?
[Randomwarlock] has invited you to join a group.
You have declined [Randomwarlock]'s group invitation.
[Guild] Lizzy: Okay!
You whisper to [Randomshaman]: Sorry, going to Scholomance.
[Friendlymage] has invited you to join a group.
You whisper to [Randomhunter]: Can't, heading to Scholomance.
[Randomshaman] whispers: pls!!! will pay 5g.
You whisper to [Randomshaman]: I can't -- I'm already committed to going to Scholomance.
[Raider] whispers: Hey, we need more priests for Razorgore, wanna come?
[Randomshaman] whispers: 10g?
You are now ignoring [Randomshaman].
[Raider2]: We need healers for BWL!
You whisper to [Raider]: Sorry, busy atm.
[Raider3] whispers: BWL?