The right of a practitioner to withdraw from employment, once begun, arises only from good cause. The desire or consent of the client is not always sufficient cause for withdrawal. The practitioner shall not abandon the unfinished task to the detriment of the client except for reasons of honor, or the client's persistence over the practitioner's remonstrance in presenting frivolous defenses, or the client's deliberate disregard of an obligation as to fees or expenses. In these cases, the practitioner may be warranted in withdrawing after due notice to the client with time allowed for the employment of another practitioner. Other reasons for withdrawal might include instances in which a practitioner discovers that his client has no cause and the client is determined to continue the cause, or the practitioner's own inability to conduct a case effectively. Upon withdrawing from a case, the practitioner shall refund any part of a retainer which clearly has not been earned.