Rather than give my opinion, because I am an EMHC myself (who has some questions about the practice), I submit to you a quote from Francis Cardinal Arinze, the Prefect for the Congregation for Divine Worship and the Discipline of the Sacrements. This is how he addresses the issue at hand:
It’s very important to stress that, because some people do not understand the whole point of these Extraordinary Ministers of Holy Communion. Some people think that it is a power struggle for lay people to prove that what the priest can do, so can they. Which means if there are not many people to receive Communion – suppose there are three priests or two priests and one deacon and 100 people to receive Communion – then there is no need for even one single Extraordinary Minister of Holy Communion.
…So the full answer to the question is, if the communicants are not too many and the ordinary ministers – that means priests and deacons – are enough; the question of Extraordinary Ministers does not arise. Therefore, there is no question of choosing them.
Now, here is what Redemptionis Sacramentum says regarding the issue of Extra-Ordinary Ministers of Holy Communion:
156.] This function is to be understood strictly according to the name by which it is known, that is to say, that of extraordinary minister of Holy Communion, and not “special minister of Holy Communion” nor “extraordinary minister of the Eucharist” nor “special minister of the Eucharist”, by which names the meaning of this function is unnecessarily and improperly broadened.
[157.] If there is usually present a sufficient number of sacred ministers for the distribution of Holy Communion, extraordinary ministers of Holy Communion may not be appointed. Indeed, in such circumstances, those who may have already been appointed to this ministry should not exercise it. The practice of those Priests is reprobated who, even though present at the celebration, abstain from distributing Communion and hand this function over to laypersons.258
[158.] Indeed, the extraordinary minister of Holy Communion may administer Communion only when the Priest and Deacon are lacking, when the Priest is prevented by weakness or advanced age or some other genuine reason, or when the number of faithful coming to Communion is so great that the very celebration of Mass would be unduly prolonged.259 This, however, is to be understood in such a way that a brief prolongation, considering the circumstances and culture of the place, is not at all a sufficient reason.
The problem in my diocese is that we are being over-used, especially when the number of communicants is enough for the priest to handle by himself. Now, if the parish is distributing Holy Communion under both species, then, there is a need for us because the priest (unless he is distributing via intinction) can’t very hold the chalice, the ciborrium and distribute,.
However, even during daily Mass, I have seen the celebrant call for an EMHC to help him even when the communicants number under 30. I had a serious talk with my best friend (who is my parochial vicar) because he tends to do this. I told him that even though he’s being gracious in asking an elderly woman to help him, the well-meaning gesture doesn’t ustify what he is doing. I told him to read Redemptionis Sacramentum (he has my copy because, oddly enough, he wanted to see a few items related to EMHCs). Even though we got into an argument, he managed to see the point. He still does this on Sunday evenings and I told him that he can’t just deputize someone on the spot, especially if the person doesn’t have a clue as to what to do, as we experienced when he called the ushers to help him distribute the Precious Blood. The poor guys were confused and the pastor wasn’t too happy about it.
My pastor is having us read Msgr. Champlain’s book on EMHCs. The book is full of errors and misconceptions, even though the author claims to have brought it up to speed to cover the revised GIRM and RS. There are still some questionable things that don’t jibe with the documents of the Church. Occasionally, during our training, I have to address those issues.