Great question, and your suspicion is correct: including some flatter running into your training each week will provide several advantages:
– You’ll gain improved economy of movement (cadence, force production, and stride length) which will increase your “cruising” speed. This will translate into steeper running by providing a greater buffer between what pace you can confidently run and what you’re realistically doing in the steep terrain.
– Flat running, and more specifically running on a smooth (paved or dirt) surface allows you to focus in on technique. Technical trail running is by nature inconsistent as you move over roots, rocks, and adjust speed constantly to deal with changing terrain; in contrast, running on a flat or slightly rolling smooth surface is consistent, providing the means to spend many minutes in a consistent form/technique and to iron out inefficiencies.
– Recovery: even though it sounds like you’re handling the high volume of vertical training well, it is still a big load on the body. Using flat runs for recovery (such as a 30min easy run following strength workouts) helps offset the significant muscular load of steep trail running while still providing volume to your overall base.