Is there any benefit from wearing a nightguard for TMJ if I'm not a teeth grinder?

TMJ. Some people wear one to prevent cheek biting. Also it can serve as a orthodontic appliance to stabilize the teeth .If you don't need it there is no sense wearing it.If you are having joint pain it may also balance your bite and eliminate torque on the joint, relaxing the muscle.
Maybe! Tooth grinding at night is called bruxism, and is much more common than often thought. You may be doing it without knowing it. A properly designed nighttime splint can be used to treat a variety of temporomandibular disorders causing pain or dysfunction, and may be prescribed or fabricated by a dentist or oral surgeon. However, if you have no symptoms, you probably don't need to wear one.
May be surprised. You may grind your teeth at night and not even know it. Grinding/clenching is not necessarily a symptom of tmj/tmd.
Unclear. If you don't grind at night, perhaps not. You can test it however. If wearing it at night decreases your symptoms, then you probably are grinding at night.