Over the weekend I finally saw the season finale of Arrow season 5. I used to love the Arrowverse, but as it went on it became more of a chore to keep up without the full payment I expect from my entertainment options for that much effort, but I heard good things about Season 5 and wanted to watch it so I did. I'm not going to go into details about Season 5 of Arrow, but the finale features several sequences where one or two named characters rip through groups of masked, genericked people. It's done to show that the named characters are bad ass, and whether poorly or well executed it's not an uncommon thing to see in games. However, is it a good thing for your game?
What Is A Minion?
A minion is a mechanic for a simplified NPC that a lot of games use. The key use comes in combat, where a minion doesn't pose much threat as an individual - but may be dangerous in groups - and is easy t take down - especially when isolated.
Their job is to represent the rank and file. It lets the bad guy - or opposition - have numbers on their side, but those numbers can be taken care of with relative ease by the PCs which lets the PCs feel powerful. The other thing it does is give a clear break between Important NPCs - NPCs with full names, HP bars, and character abilities - and, well, unimportant NPCs who are there to play a cardboard cutout role (i.e. masked man #6 in combat sequence 2.)
The Gritty vs. Heroic Scale
The answer to whether or not you want minions in your game - i.e. will they work well in your game - frequently depends on how gritty or heroic your game is going to be.
With grit I mean how nuanced combat and action encounters are going to be. Think of a Tarantino movie. In general in his movies, any interaction with someone who has a gun can be a life or death endeavor. With rare exception (namely Kill Bill where Tarantino was doing a specific genre that featured it) you don't see much in the way of mooks. On the other hand, Action/Adventure movies like Star Wars will frequently have numerous faceless mooks that the hero cuts down without much risk to themselves and serve as little more than a distraction or obstacle to be overcome while something else is going on.
By heroic I don't necessarily mean that the PCs are good guys, or that they're fighting the good fight. I just mean that they are 'more' than the average person and expected to be so. In a world of every day people they're the Jason Bourne, James Bond, or Jack Bauer type person.
How To Use
Once you've decided you have them, the best way to use minions is to add flavor to other things. Minions can add extra levels of threat to combats with villains. Minions can add extra danger to a scene about getting someone out of a busy area, or stealing something. Minions can be used when you need to make your PCs feel powerful, capable, and competent in the game world.
Essentially, the minions should never be the focus of a scene. If you have a PC and minions in a scene, the focus needs to be something else. Either the PC themselves - this is how capable this PC is! - or something the PC is doing.
How to Not Use
Minions should never be the sole challenge the PC(s) has to overcome. If all you have is minions and PCs, it's going to be a wash so just give the PCs a win - again, unless you're trying to make your PCs feel powerful.
Minions in moderate or small numbers should never be a threat on their own. They can add to the threat of something, but they shouldn't represent the bulk of it. Yes, there are exceptions to this rule, but don't go breaking it until you understand why it's the rule.
To put it another way, minions make a wonderful spice for certain types of games. Games where the PCs are supposed to be 'more' than other people, or special in some way work best with them. As a spice, the minion should never be the bulk of the meal, but they should add to it. Used well, your PCs will feel strong, but also have an enhanced sense of danger to encounters. Used poorly, and your PCs will either be overwhelmed...or just bored.