The idea behind "Bring the player, not the class" is that raid stacking shouldn't be as big of a deal as it was during, oh, say, Sunwell. For each buff and debuff, we have a few different classes that can provide it, so raid leaders don't have to go too far out of their way to get good coverage.
However, what single buff was the biggest factor causing guilds to stack a particular class in Sunwell? Ten points if you said "Bloodlust/Heroism." And that is, irritatingly enough, one of the few remaining buffs that no other class has; if you want Bloodlust, you need a shaman, period.