I cant speak of how it plays out in higher lvl bgs, but at lvl 19 people can cast infinately and never run out of mana. Breaking a shaman's totems repeatedly to drain their mana is a viable strat at lvl 19 in vanilla and offers counter play to classes like Warrior going into a shaman. however due to mana reduction many tactics revolving managing the enemy's resources are completely pointless.
The only people I have seen need to sit and drink are people who were just spamming heal..
So in a team fight as a warrior I am constantly starved for rage whilst they have insane uptime on their casts.