The purpose of the auxiliary bishops is to assist the bishop in the running of his diocese. If the diocese is large and there is a large number of people, more than what one bishop can handle then the auxiliary bishop is there to help him, not sit in a corner with nothing to do. Canon law assigns them specific functions to perform in assistance to the bishop in question.  

The auxiliary NO bishop in my area seems to have nothing better to do than comment on the latest hollywood movie that has come out.  It is my guess that these degrees of bishops is a more recent phenomenon(past 1000 years vs preceding).  I can see the importance of a nuncio, and even archbishop for major cities.  But, outside of that, I think it is a net negative.  It seems like a grooming technique.  Groom the bishop to be submissive by starting them early.  

Where I live the auxiliary bishops are busy doing confirmations, special masses, etc...  

The reason I think it is argued to be occurring is to ease transitions from priest to bishop and so on.  But, that is flawed.  Because, the sacraments confer sufficient grace.  If a diocese is too large for one bishop to shepherd it, then the size needs to be changed.  Because, this concept of having multiple assigned bishops to a diocese seems faulty, and inline with collegiality.  And we think collegiality came out of nowhere.  If it is a problem, and it is, then you not only have to reject it, but you have to reject its near occasions, especially when they are not traditionally traced back to the deposit of the faith/the apostles.