Hip flexors, at least in my short experience, are very useful, but only in motions that require a quick drive of the knee. (HURDLES). In hurdles, for example, without sufficient hip flexor strength, the trail leg doesn't move forward fast enough over the hurdle and it delays the time it takes for the lead leg to make contact (this part of the motion being highly dependent on hamstring strength). They're definitely useful, but crazy hip strength isn't really necessary for most athletic movements.