Not being a pro, but a accomplished amateur of the E Wheel.
This is not the correct answer, but!
I have found that the flat anvils are ideally suited for the beginner or amateur that hasn't had much experience with the wheel to start off with,
they are a bit more forgiving if your lazy and not consistent with your tracking and pressures.
With full radius anvils your tracking has to pretty tight with the correct pressure, A beginner would find it difficult to get his/her grip on, by trying not to leave marks all over the panel.