If I remember correctly cant you buy voidstones in all modes (Campaign, Besieged, Conquest) right. Simple solution to me would be to just do campaing battles for Xp/an and buy them there. At max evaluation i get 9000exp/4500an per evaluation. So if I remember correctly isnt that at least 2 void stones per battle so what are you complaining about? Also eliminating voidstones would break the voidwatch system and allow people to monster hold which i imagine is defeating the purpose of the voidstone in the first place.