Spirit is useless to Spriest; you only get 15% of mana regen while casting, which is basically nothing.
In vanilla most fights are really short, 3 minutes or less, and Major Mana Potion + Dark Runes are enough for that. For longer fights you just downrank your spells a little to adjust. Remember that you're not really there for your DPS, you're just there for Shadow Weaving.
As for leveling, you really don't use your damage spells all that often, you really just use SWP + wand for the most part, and you'll have almost no drinking time at all even when not stacking spirit. Lots of cloth gear will have spirit on it anyway so you don't need to worry.
I'm super experienced with vanilla Priest so if you want a giant textwall of information, just PM me.
Every raid needs 1 spriest to buff the warlocks with 15% extra damage. The more warlocks you have, the stronger a shadow priest becomes in terms of overall raid damage.
Stop spreading misinformation.