Hmm, interesting question. I'll certainly be running Shadow and Disc as my two specs. 95% chance I'll run as Shadow if I'm solo questing, but if it seems to be a consistent problem to stay alive, I can always swap Disc =] I don't think Holy will have any place for questing / leveling personally.
I'd probably recommend to anyone without a Shadow spec to pick up one for leveling and have a second spec for healing for the dungeons. And if you're a pure healer, drop Shadow at 90. I've leveled two priests (leveled my first one as Shadow, second one as Disc) and between 80-85 on both, I found Shadow to be more convenient than Disc, so I'm assuming that 85-90 will be similar in the leveling aspect. Just my 2 cents
You can heal non-85 5 mans even if you are specced shadow.
I've walked the realms of the dead. I have seen the infinite dark. Nothing you say. Or do. Could possibly frighten me.
We are not monsters! We are not the mindless wretches of a ghoul army! NO! We are a force even more terrifying! We are the chill in a coward's spine! We are the instruments of an unyielding ire! WE ARE THE FORSAKEN!
Those who do not stand with the Forsaken stand against them. And those who stand against the Forsaken will not stand long.
In world of dual specs I don't see the reason why you should punish yourself and kill mobs with smite/holy fire.
Some players have 2x healing specs I know that, but since you won't be raiding anytime soon when MoP hits, you might as well change one of the specs to shadow and go on questing with second spec being healing one for dungeons.
Not like there is big gear difference between healing and shadow gear for priests.