The quads are very, very involved in raw squatting and deadlifting (especially squatting). There's this notion lately that the only thing involved in a squat is your posterior chain. While everyone does tend to neglect their posterior chain (until they learn better), over training it and ignoring your quads is a poor decision for raw lifting. If you want to train your quads with leg press, and they respond well in terms of hypertrophy and strength, then all the power to you.
I prefer to have my squat accessory be squats and front squats lol.