Man, if you are riding a lot you should avoid flat pedals. It should be teached to the kids as first lesson in the schools. That is my own experience. When you don't have an spd pedal system you end up working the same certain group of muscles much more than the others and it causes an imbalance which in the end effects your knees. I mean the muscles that work intensely like quads and glutes carry all the work and they get really tight at the end of the day. If you don't massage or stretch them properly, your body finds a way to adapt itself to this awkward condition. Those muscles get shortened! I don't know if it is a scientific fact but I really think that it is working like this. When they got shortened they start to put pressure on the tendons which are connected to the kneecap (patella). Now your patella is standing closer to your femur (thigh bones) and they start rubbing there even you are not actively using them. This is causing a problem called quadriceps tendonitis which is the inflammation of the quadriceps tendon. If you continue pushing yourself on the platform pedals, problem gets bigger. You can have cartilage degeneration or ACL torns. That's my own experience. Another problem with platform pedas is that you can place your foot at awkward position like they are too close to the frame, pointing inward or outward. So, people should stop discussing if clipless pedals have a performance gain or not. They are needed for health reasons on the first place. Ride clipless.