Sometimes powers conferred upon a Trustee may result in negative tax consequences if that Trustee is also the beneficiary of the trust for which he is acting. When a trustee has total discretion to distribute trust assets to himself or his dependents, the assets of the trust may be included in his estate for estate tax purposes. To avoid this, the Trustor may wish to appoint an independent Trustee or have an independent Trustee act as a Co-Trustee and limit decisions regarding distributions to just the independent Co-Trustee.
Occasionally, sensitive family situations may also dictate that a Trustor appoint one or more "special" trustees to exercise discretion with respect to certain decisions. For example, if there are sensitive family dynamics, a corporate Trustee might decline to act unless it were given assurances that a special Trustee could be appointed for the specific purpose of dealing with those family members.
Utilizing independent or special Trustees can be helpful, but may require careful planning. For example, if the individual or corporate entity you've named for those positions is not available, then you may need to think of back-ups to ensure that the mechanism you're using is effective.