Maybe other people have already noticed this and can provide insight.
Basically, some monster spawn structures can generate very nice rewards when cleared. Old Crypts in Portal Worlds can generate Adepts of Lunord or Paladins of Death when cleared. I thought it was mainly another perk of exploring Portal Worlds, but I just received a Dracolich in Ardania by clearing an Old Crypt (turn 51). And that also got me wondering about what spell reward types can come from what structures.
Anyway, I thought I'd see what others have noticed. If some structures are particulary juicy, it makes it more worthwhile to burn a summon to claim one if the player otherwise would lose it to the AI. Or when MP comes out, other human players. I thought I'd do Structure/World/Reward, and if enough people chime in, we can see if there are any patterns. So! Here we go-
Ogre Den/Ard/Life Weapon Enchant
Old Crypt/Ard/Krolm's Housecarls
Old Crypt/Ard/Paladins of Death
Magic Thicket/Portal/Summon Fire Elemental
Elemental Portal/Portal/Treewarden (x3)
Elemental Portal/Portal/Greater Fire Elemental (x2)
Dragon's Nest/Portal/Great Healing
Elemental Portal/Portal/Summon Earth Elemental
Elemental Portal/Portal/Greater Earth Elemental (x3)
Old Crypt/Portal/Elder Vampires
Dragon's Nest/Portal/Shadow of Vendral
Thanks to LeBaiton and Rash for pitching in!