Are you kidding? The splines are far more durable than aluminum threads; you'll never have to worry about them stripping. It still employs something like a lockring, but it's a steel spring circlip that makes it impossible for the cog to lift out of position.
I've never heard of a single case of a coaster brake of 3speed cog coming off while in use, ever. (though now, someone's going to jump in with some horror story...)