Ever fight Garalon in Mists of Pandaria on WoW? That's pretty much the same as a "ranged tank" fight. Although it doesn't technically have to use a tank at all, I ended up kiting as a healer. The reason dedicated tanks aren't ranged is so that bosses can move more easily. If they're ranged and the boss is ranged, it will have no reason to move without an outrageously large room.