In the early game of Diablo II I seem to be constantly facing these pesky Demon Shamans that endlessly respawn/resurrect nearby fallen demons. I recall there are similar enemies later in the game that respawn different type of enemies. I observe that the respawned versions don't seem to ever drop loot. Am I getting experience for killing these respawned enemies, or, to add insult to injury literally - are they just a waste of time?
DiabloWiki provides a good explanation:
For resurrected monsters, (by shamans (e.g., Fallen, Mummies, and Fetishes))you only get experience from killing a monster the first time.
For monsters produced by monster generators (e.g., Foul Crows, Zombies, and Imps), they are worth experience, but very little and do not drop any items. However, The generators themselves are worth good experience.
For Reanimated horde zombies, they are worth full experience and may drop items when they resurrect themselves. Each kill counts for the full amount.
Additionally, Monsters killed by another, under Convert, Confuse, or other related skills, award their experience to the character who cast the spell on them.
Resurrected minions are a complete waste of time. Dead Fallen that are resurrected by Fallen Shamans give no reward whatsoever for killing them again. The same is true of the skeletons in Act 2 who can be respawned by those mummy-type fellas.
Unfortunately, I don't know for sure how exactly experience works for enemies that spawn new enemies altogether. For example, the giant cockroaches in Act 2 can spawn baby cockroaches; I don't know whether they give experience or not.