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it's probably because whirlwind is pretty OP that they limit it.
If Whirlwind or any other skill is considered unbalanced, then limiting the availability of the spell to balance it out is a very bad way of dealing with the problem and it also limits the way the players can play.
They should instead balance/design the skill appropriately and not punish you for playing (in this case) more than one warrior as well as prohibiting you from respecing your skills, because you won't be able to ever get that skill back.
All skills should be obtainable one way or another, either through crafting or vendor sale.
