Are they talking about Classic/TBC?
Mana is the equalizer for healing. When you’re playing Shadow, you run out of mana fast when you’re spam healing. That’s to prevent a DPS class from having near infinite healing ability. If you’re just doing the main purpose of the spec (DPS), you should never run out of mana.
Come to the priest forums for some solid info, and to see what’s going on and this is the first post I see. 