[Twan]

It was the same in dom2 IIRC. Vanheim has already 4 units, including their only good cheap mages capital only, so IW may have wanted to keep them something useful to build in other forts (IMO, non capital only Dwarven Smith or Fay Boars but capital only Vans would have been better anyway).

(edit : Walkyries have glamour too, so better be them capital only)

But the best answer would be a global nerf of glamour in battles (ie : don't work out of melee or only work against ranged weapons), then all ***heim nations won't be overpowered.