My guild shifted from shadow priests in healer groups to shadow priests in mana-based dps groups on most fights. If we had a third raiding shadow priest we probably would give that to healers, but at the moment we run with 1-2. The bottom line is that if you put a shadow priest with a healer, all you do is replace potion timers with VT mana. But if you put a shadow priest with DPS, you free up the potion timer for dps potions (Destruction Potion, Haste Potion) and/or enable more aggressive casting.
As such, given that Super Mana Pots are what they are, a shadow priest will not increase healer throughput nor will it have a huge impact on healer sustainability, whereas you can extend both DPS sustainability and throughput in a caster group.
I would agree that Affliction locks are low on the totem pole, but with regards to hunters, if you have a BM hunter handy they work pretty well even mixed in with casters in a shadow priest group. Increased VT output along with spells buffed by Ferocious Insp certainly doesn't hurt. They do need it less than other hunter specs though given the typically less intensive shot rotation.