Just curious, but the fact that three pieces of frost badge gear that are also 'plate' have bonus armor, that helps DK tanks even more than other tanks, correct?
Maybe the 'bonus' armor though doesn't get multiplied, because I couldn't correctly calculate my armor upgrade from switching pieces.
To answer your question...
No. Bonus armor is not multiplied. If the item's base armor is 2000 and it has a bonus of 200, every class benefits from the bonus armor identical... it is worth exactly 200 more armor regardless of class.
Also, Warriors get as much AP out of armor as DKs (on average).
But other than that it shouldn't matter.
The only difference is that DKs are slightly more sensitive to base armor (and thus ilvl) of items.