Transition is a valid thing to train too.If you look at it, why are boxers so good at empty hands? Because they specialize in it.
Why are BJJ folks so good at grappling? Because they specialize in it.
Why do MMA fighters train kicking, boxing, takedowns and grappling combined? The specialist is BEST in his domain, but in order to move from one zone to the next is itself a special skill. Like how a boxer can set up a takedown... that's a skill in itself! And it isn't taught in either boxing or grappling by themselves ....at least it wasn't common before MMA was a thing.
The debate over specialists vs. generalists is really, really old!