I am sure this has been covered in many forms so please excuse me for starting a similar thread.
I have found various benefits of using unilateral leg weights over the big bilateral exercises such as squats and dead lifts. With the 100m in mind I would not replace bilateral exercises with unilateral exercises. Ultimately I feel they do not produce the same CNS drive or power at the joint. Unilateral movements would be supplementary here.
However, for 400m training does ultimate power matter? For the 400m strength training may be sufficient with unilateral weights alone, and by removing bilateral exercises the athlete may have more energy in reserve for running and injury potential may be lowered.
Are there any top 400m sprinters who do not use the back squat and other big movements? I know Iwan Thomas often left out legs in the gym.