Will repeated adjustments wear out the joints?

No. Actually the opposite is true. The joints of your spine are enclosed by a capsule filled with lubricating synovial fluid. If the joints are not moving properly then the fluid does not flow around the joints, starving the tissues of the nutrients that are carried by this fluid. This causes degeneration and premature arthritis. Adjustments restore proper motion allowing the fluids that carry nutrients to circulate around the joints. Chiropractic adjustments also remove nerve irritation caused by spinal misalignment. When you have a misalignment the nerves that exit the spine in that area are irritated. This causes the soft tissues in that area to contract, restricting blood flow. The lack of blood flow also starves the tissues of oxygen and other vital nutrients that the tissues require. Again, this causes premature degeneration of the tissues.

Chiropractic adjustments remove the nerve pressure and restores the blood flow allowing your body to heal the damaged tissues. In summary, if you allow spinal misalignment to persist then you are going to experience premature degeneration of the spine and possible damage to other organs and tissues related to the spine (by the way, all organs and tissues are related to the spine through their nerve supply).