Dr as it currently stands in the game is not meaningful to any great extent due to how hard mobs hit for in high end content, PRR far outweighs it and the enhancements for Dr would need to be increased significantly to make it worth while for investment of points in my opinion.
Alternately as they have diltuted PRR away from armour and onto many objects, giving heavy armour a significant DR rating would make people think about choices and make armour meaningful again.