While the most efficient use of kettle hops may be in the whirlpool additions, I submit that additions at different times tend to enhance and punch up certain aspects of particular hops. A great example is Simcoe. An addition at 20 minutes will give a nice level of earthy pine while the whirlpool additions really accentuate the mango/tropical flavors. And when balanced well, the dank pot flavor comes through nicely, too. It gives more dimension and complexity to the hop flavor overall as opposed to whirlpool only which can get a little cloying with some hops, especially if the malt flavor is heavy or is under-attenuated.
Of course, flavor is always enhanced by a good dose of dry-hopping to bring out the aroma to the fullest.