I don’t think bounding/hopping per se is bad, but it takes a fair degree of skill and a high strength-to-body weight to execute correctly and many people go straight to bounding on the track to try to elicit greater stride length and quicker contact times from the outset. Starting off on a grass incline or wooden stadium benches allows the athlete to develop both of these with out exposure to the extreme impact of a flat surface.
I agree with Charlie that plyo’s aren’t essential, but I also know that some sprinters benefit from them greatly and suffer with out them in their programme.