I believe that the primary purpose of windshield heat is to deice the windshield - everything else is a secondary benefit.
I would not be so quick to dismiss the benefit of windshield heat for defogging purposes. When descending into hot, humid environments (for example, near the ocean in tropical countries), condensation often builds up rapidly on the inside of the windshield. Windshield heat is very effective in preventing (or removing) this condensation.
As for birdstrike protection - well, my Twin Otter has windshield heat, but there is no mention in the AFM for turning it on for bird impact protection. Might have something to do with the fact that the Vne is 170 knots. Besides, most Twin Otter birdstrikes occur on the rudder trim tab, when the bird is attempting to overtake the slow-moving Twin Otter.