i strongly and emphatically disagree with this widely held opinion. weight training is based upon isolating certain muscles and strengthening them. this is the OPPOSITE of learning whole body coordination as we do in aikido. IMHO best way to help body stay 'together' is by learning how it works - through aikido, or tai chi, etc.
Are you just stating this as an opinion? Because my understanding of medical fact is that when you strengthen your muscles you end up tightening the ligaments connecting to the joints, thereby, protecting them. This is especially relevant related to you knees. Maybe some of our medical experts here could explain it better than me.
Learning whole body coordination is one thing, and training in aikido itself conditions our bodies to doing aikido, but if a person doesn't have the necessary physical strength to maintain the abuse we put our bodies through then we can suffer injury, or at least be more prone to it.