It's upstaging the Holy Sacrifice and not adding to it in any way.
In the Classical and Romantic era (from 1750-1900 and composers like Mozart, Beethoven, etc.), the music used at Mass began to get exceedingly bombastic, adding brass, strings, and
. And it was being performed as liturgical music, especially in Mozart's native Austria and in Italy. But most of it (90%) has no place in the Mass because it was distracting from the Holy Sacrifice and turning it into a spectacle.
This is why St. Pope Pius X released Tra le sollecitudini
in 1903 and mandated a worldwide return to Gregorian chant because it accompanied
the liturgy and didn't overshadow
The schola director is overshadowing
the liturgy by trying to show off with this practice. Even then, the people cannot understand his direction. The average trained musician will get about 4 weeks exposure to Gregorian chant, and even then, none of it is singing it or learning to read it -- simply the beginning of Western liturgical music and how important the Church's chant was in that role.
But your average untrained musician will not understand an arsis or thesis (conducting chant and its stress). That would be the equivalent of asking a native, monolingual English speaker to read Tolstoy in Russian. They'll get it by ear. They do not need this pretentious loon in the front acting like he knows what he's doing.