Unless you focus on PvP, you don't need to focus on Stam at all. If you are on a PvP server, however, he's right. You don't have much mitigation, and stamina is what will save your **** from annihilation. Still, to maximise your leveling speed and reducing downtime, Spirit is definitely a good thing to get. Why? Spirit Tap. Your Spirit increases your mana regen. Spirit Tap increases this by 100%. Every point of Spirit you get count twice due to Spirit Tap.
So for pure PvE: Int > Spi > Stam. Higher level PvE: Int > Spellpower/Spi > Crit > Stam.
For PvP though: Int > Stam > Everything else. Higher level PvP: Int > Stam > Spellpower > Crit > Spirit.
In PvP, you'll rarely have the option of letting your mana regen. That is why you need the biggest mana pool out there, to make sure you have what is needed to finish the fight before you run OoM. Then increasing your Stamina high enough to be able to survive everything long enough to Bubble, Fear and heal yourself back up again.
Personally, I always stuck with Int as the highest stat for a Shadowpriest, because I really loved being able to unleash all that damage for so long, without having to rely on mana regen or taking breaks. And with VT, Shadowfiend, Dispersion and so on, you can make that manapool last forever once you get higher up in the levels.
Still, never underestimate Spirit in PvE. It's a huge part of our longevity while leveling, questing and so on. Anyway, don't care about Stamina in PvE. It's useless for us, as we can just pop a Renew/Flash Heal once in a while. Focus on Mana, Mana Regen, Spellpower for PvE, and leave Survivability stats for PvP.