Let me make a suggestion. Could it be that Benedict, who is well known for his encyclopedic knowledge of the Church Fathers, is taking a page not simply from Gregory XII, the last pope to resign (1415), but also from Gregory the Great (540-604)? I have yet to see anyone point this out, but consider the following.
Gregory the Great wrote what is considered the classic treatment on the spiritual formation of pastors, The Book of Pastoral Rule. Benedict clearly recognizes the importance of this work.
During the series of Wednesday audiences Benedict devoted to the early Church Fathers, he had the following to say about Gregory's Pastoral Rule,
Probably the most systematic text of Gregory the Great is the Pastoral Rule, written in the first years of his Pontificate. In it Gregory proposed to treat the figure of the ideal Bishop, the teacher and guide of his flock... Taking up again a favourite theme, he affirmed that the Bishop is above all the "preacher" par excellence; for this reason he must be above all an example for others, so that his behaviour may be a point of reference for all... Nevertheless, the great Pontiff insisted on the Pastor's duty to recognize daily his own unworthiness in the eyes of the Supreme Judge, so that pride did not negate the good accomplished. For this the final chapter of the Rule is dedicated to humility: "When one is pleased to have achieved many virtues, it is well to reflect on one's own inadequacies and to humble oneself: instead of considering the good accomplished, it is necessary to consider what was neglected". All these precious indications demonstrate the lofty concept that St Gregory had for the care of souls, which he defined as the "ars artium", the art of arts. The Rule had such great, and the rather rare, good fortune to have been quickly translated into Greek and Anglo-Saxon. (General Audience, Wednesday, 4 June 2008). [source]Now check out what Gregory says about the need for the "ideal bishop" to be free from the frailties of the body:
"That man, therefore, ought by all means to be drawn with cords to be an example of good living who already lives spiritually, dying to all passions of the flesh; who disregards worldly prosperity; who is afraid of no adversity; who desires only inward wealth; whose intention the body, in good accord with it, thwarts not at all by its frailness, nor the spirit greatly by its disdain: one who is not led to covet the things of others, but gives freely of his own. . ." Gregory the Great, The Book of Pastoral Rule, 1.10 (cited from NPNF2 vol. 12, p. 7).Hmmmm... Am I crazy or is it hard to read Benedict's statement and not think of Gregory the Great's advice:
"... in today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me".