...Conductors train all the musicians beforehand but then during the performance they're unnecessary other than to stand up there to take credit for the performance and so the audience has something to look at...
There are some cues and especially timing aspects that require somebody to conduct a group. The way that the music moves ahead, on or behind the beat is crucial to how it will "feel". It is something that is not on the sheet, and that is why the conductor is so important.
Even Baroque music performances will have a musician the leads the group, as a conductor was not the custom at the time. It will usually be the soloist.
I don't really know much about chant, but I notice that some kind of direction is necessary. The rhythm can get messy if there is not a clear leader.
Of course it doesn't need to be done dramatically and theatrically.
I had never heard of "flapping" and it does sound very innapropriate. How can a person be allowed to stand on the front of the altar calling attention to itself? Does this practice have a historical precedent?