Getting FKSs will not make your knees any safer than another standard binding on the market. They all (with the single exception of Knee binding) abide by the exact same standards as far as release is concerned. People on this site seem to think that without fail, FKSs are safer than other bindings. Sure they offer a higher level of elasticity which allows you to ride at your proper din setting without fear of prerelease. However, current bindings are designed to prevent you from breaking your let, NOT to prevent ACL tears. There are two major "injury mechanisms," as they are called, that can lead to ACl tears.
1)The phantom foot. This is when, in the midst of a crash, forces act on the area of the ski behind the binding heelpiece. In the general case, a skier falls backward and catches an edge as he puts all his weight on the inside of the downhill ski. This causes a moment to act at the heelpiece, but since there is no lateral release mechanism there, the ski cannot release, and the forces "travel" (for lack of a better way to explain it) up your leg to the next easiest way to diffuse the force: your knee ligaments.
2) Anterior drawer. Usually occurs when a skier lands super backseat off a jump. As he lands, his legs straighten out instinctively, resulting in a landing on the tail of his ski. The ski, in combination with the stiff boot, causes a moment about the axis in line with the lower leg. Since most bindings have no way to sense this kind of force, the binding will not release, or will release too late. The force pushes the tibia forward in relation to the femur which tears the ACL.
Skiing FKSs, or ANY other "regular" binding on the market for that matter, simply is not designed to react properly to the types of forces involved in ACL tears. It is a very common misnomer that bindings are designed to save your knees. They are not. Not FKSs, not Markers, not STHs, not any other standard binding. The design intent is purely to prevent lower leg fractures. So sure, using FKSs will potentially be a little safer in terms of less risk of broken bones, it will do nothing to reduce risk of an ACL tear. Unfortunately, the only solution out right now to combat this type of injury by reacting to the ACL tear injury mechanism forces is the Knee binding.