None of my players have assassin characters.
You don't have any Assassin characters at your table because checkmate rules make them obsolete. Who would choose to be an Assassin when they can get a better benefit through house rule? That's my problem with that houserule. When a Houserule grants abilities normally only granted to a specific class, but but gives them to anyone who wants it, that is a problem. If they wanted to be able to "checkmate" someone and kill them in one round, they should have been a Rogue Assassin. Just like if you want to be able to learn every Arcane spell in the game, they should be a Wizard. Don't just give Spellbooks to Bards and Sorcerers and Warlocks and let them learn every spell on their list. That's a huge part of what makes a Wizard different. If a certain class feature is important to you, you need to take that class.
I’ll agree there doesn’t need to be a special checkmate rule. Certainly not in every game at every table. You reckon I might have a reason to use it in my own home games,
or nah?
The only reason I can see for having a "Checkmate" rule is if the group is so powerful, rolling is just a formality. TECHNICALLY, they could roll 1's the whole time and miss every attack, but that's not likely to happen. If it's a group of lvl 20's coming across a low level Orc camp, yeah, the Ranger can just say "I one shot the orc" because he, in all likelihood, probably would, it's almost a forgone conclusion, and rolling will just slow down the game. If the Assassin wants to sneak up on a dude and slit his throat and the guy is a lower level bandit or something, sure. Don't even need to roll. They've got a +17 Stealth Check, +13 to hit, auto Crit and can force a con save or double the damage from the crit. He's dead many times over. His children's children's children should die from the splash damage. Why bother rolling?