The OP isn't trying to justify ice mages (or any other build) in the current game, they're defending the concept of customizing our battle gameplay with features like talent tress, sub jobs, merit points, etc which FFXIV is lacking. You're making a valid argument, but you're in the wrong discussion.
Of course ice mages, as the game is now, are terrible, but if the game were designed around the idea of customizing your gameplay between ice mages, fire, lightning, or whatever, even if ice mages were slightly less practical many of us would still enjoy having the choice.