Expose Armor and 5 stacks of Sunder Armor grant equal amounts of Armor Reduction. With Improved Expose Armor, a rogue can apply Expose Armor more quickly. For this reason, Expose Armor is a better debuff than Sunder Armor in terms of raid wide damage gains.
Expose Armor and Sunder Armor don't stack, though, so a Protection Warrior might lose a certain amount of damage from his Devastate ability as it gains additional damage from each stack of Sunder Armor.
If there is a tank different from a Protection Warrior tanking, applying Expose Armor definitely makes sense.
/Patch 4.0.1 (12-Oct-2010): Now gives a chance to refund all combo points when performing Expose armor. Previously reduced the energy cost of Expose Armor.