If every time you turn your head, your right arm goes straight, you’re in trouble. If you’re driving a car at that point, you will veer into traffic. The biggest problem is that it stops you having control over your environment. You don’t have a choice.
Developmentally speaking, primitive reflexes have to first teach you to move then they meld in an appropriate fashion. When correctly done you can have a choice of two responses or three, later on when you experience specific sensations. Like turning your head or walking on uneven surfaces. Hence they become part of the nervous system. The pathway of sensation through muscle contraction still exists, but it’s not an obligatory pathway. Baseball players use the response pathway when their arm straightens to get speed on the ball because it’s a pathway they don’t have to think about to use. You use it when you stand on a tack, your foot jumps up, but you keep balance on the other foot. So your one leg goes straight, and your other one comes up. They are safety mechanisms. They “run” as it were and they are there forever. They do not disappear, but the point is that when you’re an adult, they should be there to keep you safe. They provide an unthought through response that you can do immediately if you need it but you should also be able to choose whether you want to use it or not.
Whereas when it’s inappropriate, you can’t choose whether you use it or not. You don’t have a choice; it has to happen. The stimulus/sensation was perceived, and the muscles have to contract.