SwordLord
Attack!
And there in lies the problem. Hammerfall has zero crossover appeal to the Prog crowd. And they're at such an extreme end of the Power Metal spectrum, that their cheese factor is too high for even some Power Metal fans.
I can defeat your theory with two words ...
VIRGIN STEELE
Score one for me!
Obviously it's all a matter of taste and it's Glenn's fest so he can do what he wants, but I would love to see Hammerfall at least play a 4th tier slot eventually. If they aren't strong enough to headline, they are at the very least strong enough to appear.