whirlwinds are really good at taking out anything in carapace armor (and i do mean storm troopers nobz and aspect warriors).
remember if you don't mind scattering i.e. playing an ork horde fire indirectly then the cover save is worked out from the center of the template so hiding behind other units doesn't help. and at 85 points they are such a great choice in small games.
another advantage of being 85 points is its not worth a lascannon to most people if you've got a few other heavier tanks and transports being a bit scary you will find your whirlwinds make it the end of the game when your vindicator did not.