When/how much do we get experience from spawned enemies?

I noticed that I did not get any exp for the skeletons spawned by the Activated Pillars (during the quest A Shattered Crown). Not sure if this was right from the start or I get xp for the first few spawns killed - I was trying to test out a comment on using level up blast to topple all 4 pillars for the achievement, when I saw that I get 0 xp for letting more skels spawn and then kill them :(

Do we ever get xp for killing such spawns, or its always best to just kill the nest or pillar asap? If they give no xp, is it better to just avoid all the plague crow nests etc. in the wild instead of wasting time on killing them (or do the 'nest' give more xp to compensate)

You get experience for the initial group of enemies, but any new spawned enemies give you 0 exp

This was probably done to prevent exp farming, since Blizzard wants you to go out and explore their world, not sit in one spot and farm experience :)

Edit: The "initial group" includes the monsters that are alive with the spawner too. So if you walk up on one Crows Nest and 4 crows, the 4 crows will give exp but any new crows spawned won't