Are these people right? Let's take a look at what JPII, quoting Pius X, had to say on the matter:
12. With regard to compositions of liturgical music, I make my own the "general rule" that St Pius X formulated in these words: "The more closely a composition for church approaches in its movement, inspiration and savour the Gregorian melodic form, the more sacred and liturgical it becomes; and the more out of harmony it is with that supreme model, the less worthy it is of the temple"
In short: "New compositions on the Gregorian model? Great idea!"
The adaptation of of Latin chant to English texts was only part of MacMillan's article, and he regards it as a way of maintaining continuity with the past. As one commenter over at Fr. Z's said, most people who use this method now are using it as a bridge to try and bring chant back, and getting the congregation used to hearing our traditions once more. I know this to be true in at least one case, for that is what I myself am doing.
In the months I have been cantoring off and on at my parish I have introduced using the propers, in English, for communion. I could have done it in Latin, but few people use missals these days, and I thought it important for people to hear and understand the texts. But when I was asked to fill in and sing for All Soul's Day, I decided to try and go a step farther and asked if we could toss out the opening hymn entirely and go with the proper Latin Introit, Requiem Aeternam. It went well, although it did seem to throw the priest off a bit. True story: he was new to the parish (he came to Toronto from Africa to study) and this was his first Mass. He started using Latin in some of the parts of the Mass. "Dominus Vobiscum!" Apparently he just assumed we used Latin here. It was great.
So when I read these armchair critics panning without even understanding McMillan's point, I got a little defensive, because I am getting sick of this. Does Catholic music today stink? It absolutely does. But you know what? I'm doing something about it. Something real, something concrete, something more than leaving comments under a pseudonym at random blogs. MacMillan is also doing something. Complaining may be fun, but it changes nothing in the end. You want to make some changes, get off your duff and get in the game. Find a way to support your music program even if you can't sing or play. Encourage and give whatever aid you can to those who do. Do something other than pointlessly whine about those who are putting themselves out there and making an effort. Otherwise, shut your mouth.