Rose has improved defensively, but he remains the least valuable defensive player in their stating five ... which directly relates back to the talk of Most Valuable Player.
As is the case with most great players, what Dwight does on the offensive end doesn't just show up in his stats. He dominates games not just by scoring, but by drawing so many extra defenders into the paint that his teammates stare down wide open three after wide open three. But the same holds true with Rose, as his offensive value doesn't just show up in his stats either, and I agree that Rose holds more value on that end of the floor than Dwight.
But the gap separating Dwight's defensive value with Rose's is significantly bigger than the gap separating Rose's offensive value with Dwight's, which is why I think he deserves the award.