I just received a query from a person who is dealing with a microphone controversy in her parish. Her director of music is very much against their use but parishioners themselves complain that they can't understand the words being sung in this very live environment (7 second echo!) with a tall dome. As I read this, I found myself very sympathetic with the music director. After all, the Christian liturgy somehow managed to go about 1,950 years without the use of microphones.
In the balance, I don't think it can be said with certainty that the net effect of microphones has been good. Too many churches have been built with the presumption that electronic mics and mixing would be the norm -- and hence there's no problem with flat roofs, carpet, and sound tiles. This environment is a mortal enemy of chant, which requires not a studio-style acoustics but the liveliness of stone, concrete, wood, and angles.
It's truly tragic when parish wants to move from pop music to liturgical music only to discover that the chant ends up as flat and dead in the space. Without microphones, the singers have to scream and they lose all subtlety in singing. Without microphones, the chant comes across as just another form of the same type of music that had been there before. The only real solution is to rip up the carpet and tear down the sound tiles.
But what about a situation in which the sound is actually too live to the point that the text can't be understood at all? After all, the purpose of microphones is not just to make the music louder but also to make it more defined and clear. This is really a matter of the specifics of time and place. I can easily imagine how microphones could help in this case, and I see nothing somehow outrageous or contrary to the faith in using them.
Certainly in the case of a carpeted and acoustically dead space, using microphones and accompaniment on chant can be a beneficial thing. Even more than that, it might be essential. So long as we understand the goal -- sacred music that sounds as beautiful as possible -- the use of microphones is really up to the discretion of the singers in consultation with pastors and others.