Usually I am firmly on the "PvP gear BAD!" bandwagon, so it all but physically pains me to admit this, but the math does indeed support, in specific cases, that using PvP gear, for bonuses such as the DK 4-set (+250 Strength), outweighs using four pieces of PvE gear in those slots.
For the most part, and in my opinion why people refuse to let this one go, your average player encountered in the Dungeon Finder in PvP gear, has not done the math and is, in fact, using it to cheat the average ilevel requirements. As for the situation arising where PvP gear can be better for PvE than PvP gear; I find it to be lazy development work by the staff at Blizzard. PvP gear is intended for use in PvP encounters. PvE gear is intended for use in PvE encounters. If the two are interchangeable, it defeats the purpose of having the delineation in the first place.