Since the cantor thread seems to be dead at this point, I thought I would post anew in order to thank everyone for what turned out to be quite a constructive conversation about the use of cantors in the Roman Rite. Many of your comments will occasion a revision on my part if this piece were ever to be printed elsewhere. For now, however, this "appendix" shall suffice.
Special thanks to those who mentioned two uses of the cantor which had evaded my attention, due to the fact that I just couldn't bring myself to go through the GIRM yet again. They are 1) the cantor leading the Bidding Prayers (Prex fidelium) if the deacon doesn't do it and 2) the cantor alternating Psalm verses with the choir or congregation during the processional music. Notice carefully that these situations involve the cantor singing as an equal rather than as master.
In terms of number 2 above, while it is a legitimate option, it would not be my preferred method of singing the processional Psalmody, as this task has been assigned to the choir in the Roman Rite for centuries. Someone (or two) asked earlier how I can make this assertion. My reasons for saying this are twofold: 1) It is traditional that the choir sing the Introit, Offertory, and Communion and 2) from an idiomatic standpoint, if the Propers from the Graduale Romanum are sung, these belong to the choir by virtue of their musical properties. These antiphons are not suitable for singing by large groups of people, particularly untrained singers. (But no hard feelings...) Related to the idiomatic concerns about the Propers, we can also understand that at times the Ordinary would belong to the choir as well, that is, when it is sung in a polyphonic or orchestral setting. Musicam Sacram (1967) offered many options of who could sing which of these parts, as long as the congregation was not completely excluded from the singing. This leaves us with a wide array of possibilities, since, even if the choir were to sing all of the Ordinary and all of the Proper, the people would still have dialogues and other things to sing. (This is not the time to decide if such a case would be desirable; nor is it time to re-hash the choral Sanctus....please, please.....the very thought makes my head hurt.)
But I digress a bit....moving along...
As far as song leadership, I agree with whomever said that the organ leads the hymns. If the organ does not lead the hymns then the organ should not play the hymns. This presumes, of course, that the organist is well-trained, and that he knows how to register the organ properly (and not doing things like using tremulants and celestes for congregational singing) and to lead people without suffocating them with some of the ridiculously fast tempi that can be heard in many places today. It also presumes that the organist approaches the hymns with a singing mentality, though that doesn't necessarily mean that he must be a trained singer.
This is far better than one amplified voice doing the leading. Think about the times you've sung your country's national anthem at a public gathering. When was it sung most heartily? When a pop soloist crooned it into a microphone? Or when a band (as in a military band) played a traditional arrangement of it? My vote is for the band. Also, for more info about the harm the mic does to singing congregations, see Thomas Day: Why Catholics Can't Sing, p. 52. It is also worth noting that, in many places, the organist and cantor seem to fight for the lead, in a manner of speaking. I don't think it takes much to see how this is harmful. Also, special thanks to the reader who mentioned the sociologist's book which approached the subject of amplification, etc.
Ephrem's point about Protestants--that they're Protestant--is very well-taken. But so is the "opposing" point, which seems to be saying that we need to find out how the Protestants got to their robust singing and learn a thing or two from it. I'm quite confident--even certain--that Ephrem agrees with this, too. This is the issue which I tried to address--perhaps inadequately--at the end of my original piece. Ultimately, if we want to take the "training wheels" off our congregational music, we musicians are going to have to roll up our sleeves and teach.
Well that's all for now. And again, many thanks.