The idea behind the paladin class is tanking and healing. Most classes have two roles they can play very well given the proper spec and equipment. But Druids can play more, why shouldn't other classes?
This isn't a qq moar ret pally question or a n3rf druids question. The question I'm putting forth is: should all classes get to do all things? And should they be able to do them very, very well to get raid slots?