There are ways to do Raz without two priests--it's not easy and I doubt any pug will be bothering but it is doable.

There seems to be a very distinct effort by blizzard to encourage raids to allow people who play healing classes to fill a DPS role and be it be desirable to do so. Oomkins have the crit buff, plus optional hit increase. Spriests have misery and replenishment. Ret pallies have replenishment as well.

As much as they say "Bring the player, not the class," Blizzard also has an interest in encouraging class and spec diversity in raid groups. If shadow priests, ret pallies, or oomkins were just compeditive DPS, or only provided utility that could be obtained without sacrifice from a "pure" DPS class, there would be pressure from other classes to pigeon hole all of the people that play these classes into non-DPS roles.

In my opinion, Replenishment is perfect. Potentially a major component of later raiding, but a common "pure" DPS class has access to it with some personal DPS sacrifice. If you're in a guild that's serious about raiding, you'll work it out. If you're expecting to just slap whatever pug is together from the LFG channel past Naxx etc. I expect you will be in for quite a bit of wasted time.