Regularly running at a variety of paces is what should be considered the norm, not an exception that only runners interested in running as fast as possible should bother with. Like a cook who can master a wide range of dishes and techniques, courses and cuisines, a runner who’s comfortable at a variety of paces acquires a broad range of proficiency that makes every run better.
What I’ll call multipace training simply makes you a more complete runner. Multipace training gives you greater running-specific muscular strength, moves you through a greater range of motion, fully develops all of the internal processes that underlie running, and optimally prepares you for whatever sort of running challenge you want to take on.
I’m not saying that people who don’t train at a variety of paces aren’t “real runners,” whatever that tired phrase might mean. You can lead a satisfying running life by running exactly the same pace for the same distance every day. But there’s a whole other type of fitness available to runners who regularly rotate through several types of workouts. It’s a fitness that will make your running more enjoyable regardless of whether you ever plan to test it in a race.