When tanking I've found that while your skill rotation can maintain most of the taunts, Horn does more than just taunt in my opinion. Timing the shield and heal on your party can make a difference in maps where you can then do big pulls of mobs, while maintaining tank aggro and letting the DPS classes go all out on the mobs.
As Capeo mentioned, its often more about keeping the party alive so that they can do what they do best.