It is well known how auxiliary bishops came into existence: When a Christian community was expelled from its home – e.g. by a natural disaster or a war – the community members migrated to another place, and sometimes they found a new home among other Christians. The bishop became an other bishop’s guest and assisted him in some episcopal tasks, keeping his former homeplace as his title (titulus in Latin). His former cathedral, which had turned into a dwelling, a mosque, or just a pile of stones, remained as his titular see: a lost diocese that the Christians wanted to restore whenever possible.
Next step: The titular bishop dies. The Christians still claim their lost see, so a new bishop is ordained on the old title to keep the claim alive.
Next step: The host bishop gets used to always having an assisting bishop around him, to whom he can delegate several tasks (parish visitations, confirmations, etc.). Nobody remembers the historic reason. There are two or even more bishops in one place, one of them being the leader of the diocese, the others doing episcopal things, but somehow subordinate. This is the current situation in the Catholic Church (in many places): One diocesan bishop is the only leader of the diocese, and auxiliary bishops in the same diocese hold a titular see, often in North Africa or the Middle East.
The Second Vatican Council adhered to the principle of mon-episcopacy: Any local church is led by a single bishop. He presides over the liturgy, he teaches the Gospel, he is judge and lawgiver (within general boundaries). His office represents the apostles’ office. Monepiscopacy is supposed to guarantee and make visible the unity in faith and the unity with other local churches and their bishops.
Now if this is the theology of episcopacy, why do auxiliary bishops still exist? The Catholic Church clearly shows that it does not seriously want to restore the titular sees: Many titular sees could be restored without any problem, e.g. in countries without any prosecution of Christians, such as Germany (titular diocese of Chiemsee), Austria (Teurnia), Ireland (Roscrea), or Italy (Formiae). Over the last decades titular sees have even been invented on the base of an excavated pile of stones which looks like an ancient Christian cathedral (such as Teurnia, a lovely excavation place by the way). More than that: The 1917 Code of Canon Law (canon 348) banned titular bishops from actually restoring their titular sees! In most cases today, titular sees are just needed as legitimation for the ordination of bishops without dioceses.
A diocesan bishop can delegate a lot of his tasks to other people, mainly to priests, but in some cases even to deacons or laypeople (he does it every day, when priests preside over Mass in unity with him, commemorating him in the Eucharistic Prayer). The only task that strictly needs a bishop is the sacrament of ordination.
So why are there still auxiliary bishops? Why do diocesan bishops ask the pope for auxiliary bishops? Why does the pope fulfill these wishes? I think the answer has much to do with liturgy: Parishioners expect the presence of a bishop on several occasions, for anniversaries, for the blessing of a new organ, etc. They feel respected and they enjoy the solemnity of a Eucharist presided by a bishop in all his vestments, all the distinctive features of episcopal liturgy, and all the affectionate efforts by musicians, altar servers and all who take care of such a celebration, and all media attention around. But for most people it is not decisive who this bishop is: the diocesan bishop, an auxiliary bishop, a bishop from somewhere else, or even an abbot who is no bishop but looks like one. A diocesan bishop knows that he cannot fulfill all these expectations on his own, so he needs auxiliary bishops who give the liturgy a sort of – and I do not mean this disrespectfully – episcopal glamor.
This mindset is understandable, but in my eyes it is unadequate to the theology of episcopacy (even if the Council itself permitted auxiliary bishops when necessary). If it is a bishop’s vocation to preside over a local church, then there is no reason for the existence of bishops without local churches they could preside over. Why are merited priests virtually sent away from their dioceses to a pile of stones somewhere in the world and then virtually retrieved to their home dioceses to become auxiliary bishops, doing things that regular priests could do as well, only with episcopal vestments and ceremonies? Is it not respectless to parish priests if it needs a man in episcopal vestments to make a celebration more solemn? The parish priest already is the diocesan bishop’s representative, every Sunday, every day.
The meaning of episcopacy could be clarified if diocesan bishops abstained from asking the pope for new auxiliary bishops, and vice versa, the pope abstained from installing them.
My suggestion on the adequate size of a diocese is the following: If the bishop is supposed to be father to his priests and good shepherd over the flock (as the Council says in Lumen Gentium 27–29), he should be able to a) know all diocesan priests, deacons, and other full-time workers in pastoral care personally and have regular talks with them, b) be reachable for any of them in urgent cases within short term, c) visit all his parishes every two or three years, for visitation, confirmation, and the consecration of new altars.
The Catholic Church has about 1.2 billion members worldwide, about 500,000 priests, and a little more than 5,000 bishops, 3,000 of them being diocesan bishops, the others auxiliary bishops or retired: about 2,400 people per priest and 100 priests per bishop. If every bishop was a diocesan bishop, we would have an average of 240,000 catholics and 100 priests per diocese, without any auxiliary bishop. Does that sound reasonable? I think it does.