Questions often arise as to the usefulness of the small Velcro style attached weights. But that may be the wrong question to be asking if you are interested in safe, healthy exercise.
Attaching the weights to your ankles and doing any type of fast movements, where your legs are extending, such as happens when running or jogging puts an excessive strain on the knee and hip joints. Over time, this can cause problems leading to laxity in the joint capsule, potential constriction, and/or adhesion of the surrounding tissues resulting in abnormal wear patterns in the joint.
Thus, using these ankle weights for any type of aerobic exercise is not recommended. However, there are times when they are useful in an exercise program. And those times are when you need some extra resistance doing leg curls and quad sets at home.
These little weights are ideal when you are recovering from a lower joint surgery, particularly a total knee replacement. They will help you get back your range of motion, help develop endurance and eventually increase your strength in small increments. All of which you will need to completely recover from this surgery.
They can be useful when rehabbing your ankle from a strain if you follow your doctor and physical therapists advice on when and how to use them.
Although I have never used them when riding a stationary bike they may even be of benefit here too.