I like this idea, but would also be concerned about the summoning aspect.
I've always liked the idea of lairs that evolved. Early in the game there are more "level 1" lairs sprinkled with higher levels. At some point these lairs "upgrade" - both the mobs (either level or next tier of type) and loot. This might happen 3 or 4 times in a game so if an area was not cleared it would become very dangerous.
Another aspect of this would be to have monsters be on different factions and have them be able to patrol a bit (pretty sure one of the mods had this, MA I think) - then later in the game when the mobs are high level they get a larger patrol radius and they also clear each other up a bit.
That's been my own desire, so while related I hope it doesn't hijack.
Could you do the same thing with a random lair, something like a darkling lair. If you claim it, the mobs produced match your armor rating (leather, chain, plate, etc) or some other research or time based trigger - so as the game progresses the units are worth it.