The Rogue's Gallery

[Kevin]

Take for example a lvl 60 elite boss that has 7000 armor (I don't know how much they have, this is a total guess).  7000 = 56% damage reduction.  The warrior spams his 5 sunders on the target, reducing 2250.  Now the target is at 4750 and ~46% damage reduction.  Stack 5CP Expose Armor on top of that and armor is 3050 and reduction is about 36%.  If you had Improved Expose armor and you have a further 735 reduction for 2285 armor.  Now he has a damage reduction of 29%.  Note that 1700 adds 10% but only 45% more defense reduction with Imp. EA gives 7%.  It's not la linear benefit.  It's one of the few things in the game that small extra amounts give you LARGE returns.

In that example if the warrior also had improved sunder, it would give an additional 675 armor reduction.  This translates to an additional 7% also.
Without the benefit of the sunders Imp. EA would have only bought you less than a 4% bonus to damage.  It's the kind of thing you use in a group where there is a hunter and a warrior doing physical damage so you are all getting the benefit of the Expose Armor.
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Sunder and Expose armor do not stack and actually interfere with each other. Which ever is better will replace the other one and if the better one is applied the other will not work you get the 'a more poweful effect is active' message (however the exact wording is). The rogue would just not be able to do an expose armor of 1700 if the warrior had already gotten it up to 2250. A 1700 expose armor would prevent a warrior from doing additional sunders as well. At least this is what I have run into when playing a warrior with a rogue.