What might happen if bones have growth plates of cartilage?

Bones would be able to grow in length. Without growth plates, bones would be unable to increase in length, and people would be much shorter.

Bones would be more flexible. Cartilage is more flexible than bone, so the presence of growth plates would make bones more flexible. This would be especially important for bones that are subjected to a lot of stress, such as the long bones of the legs.

Bones would be more susceptible to injury. Cartilage is also weaker than bone, so the presence of growth plates would make bones more susceptible to injury. This would be especially true for children, whose bones are still developing and are therefore more vulnerable to injury.

Overall, the presence of growth plates of cartilage in bones is essential for normal bone growth and development. Without growth plates, bones would be much shorter, less flexible, and more susceptible to injury.