If you look at a lot of
If you look at a lot of elite distance runners (who are running very fast compared to most people) most do tend to have a slight lean. So I assume there is some value to it.
But I think it is the wrong thing to focus on while learning. I learned the lean idea from chi running (before I did much BFR) and it was somewhat confusing... I ended up leaning from my waist. But since going BF I think I have gotten more efficiency out of really focusing on staying erect and keeping my cadence high. Staying straight up also seems a bit more conservative on rough trails.
Whatever the lean is, it's SLIGHT. If you look at Danny Dreyer, the chi running guy, on youtube, he has a super straight spine, but only a slight lean.
The one place I really consciously try to lean now is while going down steep hills. I doubt I ever actually get leaning truly forward, it's just a mental thing to try to prevent leaning back and braking.