I think they are benefical – the hamstrings are a muscle group which are often injured by sprinters, so prevention will obviously be beneficial. Glute strength will also help this. In addition, I think there is some performance benefit to doing the glute/hamstring stuff you mention, which is why I do them.
I do 2-3 hip extensor exercises post running, 2-3 sets of anywhere between 4-10 reps depending on how demanding the exercise is.
What do you think is the best exercise for the hip extensors?