Objection 1. It would seem unfitting for the law to debar certain persons from the office of advocate. For no man should be debarred from doing works of mercy. Now it belongs to the works of mercy to defend a man's suit, as stated above (Article 1). Therefore no man should be debarred from this office.
Objection 2. Further, contrary causes have not, seemingly, the same effect. Now to be busy with Divine things and to be busy about sin are contrary to one another. Therefore it is unfitting that some should be debarred from the office of advocate, on account of religion, as monks and clerics, while others are debarred on account of sin, as persons of ill-repute and heretics.
Objection 3. Further, a man should love his neighbor as himself. Now it is a duty of love for an advocate to plead a person's cause. Therefore it is unfitting that certain persons should be debarred from pleading the cause of others, while they are allowed to advocate their own cause.
On the contrary, According to Decretals III, qu. vii, can. Onfames, many persons are debarred from the office of advocate.
I answer that, In two ways a person is debarred from performing a certain act: first because it is impossible to him, secondly because it is unbecoming to him: but, whereas the man to whom a certain act is impossible, is absolutely debarred from performing it, he to whom an act is unbecoming is not debarred altogether, since necessity may do away with its unbecomingness. Accordingly some are debarred from the office of advocate because it is impossible to them through lack of sense--either interior, as in the case of madmen and minors--or exterior, as in the case of the deaf and dumb. For an advocate needs to have both interior skill so that he may be able to prove the justice of the cause he defends, and also speech and hearing, that he may speak and hear what is said to him. Consequently those who are defective in these points, are altogether debarred from being advocates either in their own or in another's cause. The becomingness of exercising this office is removed in two ways. First, through a man being engaged in higher things. Wherefore it is unfitting that monks or priests should be advocates in any cause whatever, or that clerics should plead in a secular court, because such persons are engaged in Divine things. Secondly, on account of some personal defect, either of body (for instance a blind man whose attendance in a court of justice would be unbecoming) or of soul, for it ill becomes one who has disdained to be just himself, to plead for the justice of another. Wherefore it is unbecoming that persons of ill repute, unbelievers, and those who have been convicted of grievous crimes should be advocates. Nevertheless this unbecomingness is outweighed by necessity: and for this reason such persons can plead either their own cause or that of persons closely connected with them. Moreover, clerics can be advocates in the cause of their own church, and monks in the cause of their own monastery, if the abbot direct them to do so.
Reply to Objection 1. Certain persons are sometimes debarred by unbecomingness, and others by inability from performing works of mercy: for not all the works of mercy are becoming to all persons: thus it ill becomes a fool to give counsel, or the ignorant to teach.
Reply to Objection 2. Just as virtue is destroyed by "too much" and "too little," so does a person become incompetent by "more" and "less." For this reason some, like religious and clerics, are debarred from pleading in causes, because they are above such an office; and others because they are less than competent to exercise it, such as persons of ill-repute and unbelievers.
Reply to Objection 3. The necessity of pleading the causes of others is not so pressing as the necessity of pleading one's own cause, because others are able to help themselves otherwise: hence the comparison fails.
Source: Summa Theologica (New Advent)