Are 5e monsters boring bags of HPs? PROVE IT!

[Jack Bitters]

But they're not taking down their previously challenging foes in one hit. They're taking down severely nerfed foes who would die just as well if slapped by a Commoner.

Why not make them actually face those foes, rather than balloon animal versions of them?

Well let me begin by saying I've never tried using minions in my games. All I can say is what I've read and heard about the concept. I also have no real experience with 4e, from where I understand the concept of minions comes from.

That being said, I think the concept of using minions is primarily a tool for a DM to manipulate the action economy for the monsters. You only use minions when you've got some kind of boss monster--minions don't just appear by themselves. They're tacked on to solo fights to give the boss some breathing room, because a minion (due to bounded accuracy) still poses a damage threat to the PCs and must be dealt with.

When people on forums ask advice for designing a boss monster fight, one of the most common answers they get is to throw some mooks into the mix to help even out the action economy. E.g. give the evil blackguard a handful of skeletons and maybe some low-level clerics for buffs and things. The mooks have to be weak because they should not be (opinion) the main focus of the fight--the blackguard is.

Now, because of the way monsters are designed to have increasing hitpoints at higher levels (simply in order to survive long enough to have a turn), if you want to add minions that have a higher CR, you need to account for the fact that they have many more hitpoints than a skeleton or goblin or other peasant-level threat. Let's take ogres as our example. If, for example, you wanted to design a high-level fight against Ogremoch (level 20 elemental) and his ogre minions because you have a thing for ogres, you're going to run into the problem very quick that ogres have a lot of health. Roughly 9 ogres have the same hp as Ogremoch himself. That's an ogre-powered fight. But 1000 hp worth of opponents takes a while to burn through. And the problem is, level 20 characters may not be able to finish off 9 ogre minions all that quickly. A fighter might be able to wipe out one per round, maybe two. A rogue will almost certainly only be able to deal with one. A wizard will have to expend a 6th or 7th level slot to deal with the ogres in a timely fashion. But Ogremoch is the target, not the ogres. You're burning resources to chew through these bags of meat to get to the boss, but you're level 20. 9 measly ogres really shouldn't give you much pause, but they're almost as much of a challenge as the boss himself. Not good. This is where I would consider using the minion system and just giving them 1 hitpoint. Because when you're fighting the biggest baddest earth elemental in the world, I bet you wish you wouldn't have to concern yourself much with his ogre minions.

At the end of the day, it's partly a story mechanic and partly mechanical for boss fights. I would hesitate to use normally strong minions like ogres and giant scorpions except in these very high level fights where the boss should feel orders of magnitude stronger than the minions. It's a magnifying effect that wouldn't otherwise be possible because of bounded accuracy and the flattening of power from levels 1-20.

tl;dr if a minion takes more than one or two hits to deal with, then 1) you are very limited on how many minions you can use in the fight and 2) you risk having the minions be a bigger threat than the boss itself.