Best practices is for a Trustor to name one or more alternate successor Trustees in case the primary one designated is unable to act or desires to resign. It's also prudent to provide for a mechanism for resigning and appointing new successor Trustees. By naming back-up Trustees, one can ensure the continuity of management of the trust assets. Without a back-up Trustee, the beneficiaries of a Trust may need to petition the court to have a successor Trustee designated, which can be a time consuming, potentially complex, and costly endeavor.
It's important for a Trustor to remember that the successor Trustee may not be able to act when the time comes. It's possible that the successor Trustee will pass away before the Trustor or be in some other situation that renders it impossible for that individual to act. By naming back-ups you can have some control over the selection of people ultimately responsible for carrying out your wishes.