Lifting is the best thing I ever did for my damaged, pre-arthritic knees. Most of the suffering is probably not from the cartilage, but rather from weak muscles and tendons. Strengthen those, improve knee function, and the effects of arthritis are greatly reduced, in my own experience.
It's the exact same thing as people with bad backs. Bed rest may help the acute injury, but the best solution is to strengthen the problem area, not let it atrophy.
