From personal experience, I find the mid week long run to be beneficial in many ways:
1. It forces me to move forward for approximately 1:50 minutes each week. I always complete these runs on how I feel and not by any set pace. This conditions not only my aerobic base, but makes my feet and tendons strong.
2. Because of the volume and intensity of my Tuesday/Thursday workouts (15 a day on 2 runs, Tue = intervals, Thu = tempo) it keeps the paces in line and also teaches my body to learn how to run when tired.
3. Eventually the 20+ mile runs or so are only a few more minutes than your mid weeker.
I could probably think of more, but the bottom line is they make you tough.
I have always been under the opinion that a vast amount of people can train a vast amount of ways and improve. I base my training for everything around two runs. A long run of generally 20 on Saturday and a mid weeker of 14. For many “serious” runners, that is half their weekly mileage.
As funny as that seems.