A two-part article by Peter Kwasniewski from One Peter Five (Part One, Part Two) is one of the best arguments I have ever read about why "praise and worship" music is simply unacceptable for the Liturgy. Here are some highlights, with my comments in red:
If the musical style is borrowed from the outside world and brought into the temple, it profanes the liturgy and harms the spiritual progress of the people. [Remember the definition of "profane" in this context; it simply means "not sacred."]
When we hear chant, there is no ambiguity or ambivalence about what it is or what it is for; it breathes the spirit of the liturgy and cannot be mistaken for secular music in any way. [Most churchgoers these days have no problem whatsoever hearing secular music in the Liturgy, but as Kwasniewski points out later, they've had catechesis that ranges from faulty to nonexistent.]
[Praise and worship songs, even instrumentals] do not express or evoke their divine subject or the human person’s spiritual nature with appropriate musical means. The regular metrical beat and the predictable, uninspired melodies suggest a confinement to earthliness and the comfort of familiarity, as opposed to the free-floating word-based rhythms and the soaring, at times capricious, modal melodies of traditional chanting, which so well evoke the eternity, infinity, and ‘strangeness’ of the divine. [How many times has Kathy told us "chant reaches for eternity"?]
Sacred music gently moves man’s emotions in order to support and promote the intellectual activities of meditation and contemplation. This approach corresponds to the advice of the spiritual masters of all ages, who, while recognizing that emotion (or feeling or passion) has a legitimate value and place in human life, are cautious when it comes to fostering it or tapping into it for the ascent of the mind to God. Emotion is more likely to have a clouding or distracting effect than a clarifying or concentrating one; it can lead to an illusion of self-transcendence that is evanescent and disappointing. [I found myself moved to tears as a child when our "folk group" played "Day By Day" from Godspell at Mass. That became my baseline for experiencing God in the Sacrifice of the Mass, pushing aside the Real Presence of Jesus in the Eucharist, inter alia. Sacred music is about the Divine, not about How I Feel. St. John of the Cross has a lot to say about this in his writings.]Ultimately, Kwasniewski argues that with praise and worship music, the good (really, the bad/profane) has become the enemy of the best (the sacred). That is why Kathy doesn't want us to just "get by" with our singing of Gregorian chant and polyphony; we owe our best effort to the Father, the Son, and the Holy Spirit, as well as to Our Lady, the congregation, the priest, and the Liturgy.