Too kind, NT. I can't say I agree with you (apart from about the blog, obviously). It's not clear that choosing music that works better for being rehearsed, or being led by an animator, is a symptom of musicians indulging themselves. On the other hand a liturgical rendition of, say, the Gounod St Cecilia Mass, might well be worthy of that description. Actually in both cases I imagine it comes down to taste-driven antipathy on the part of the one doing the name-calling. But the way I read that excerpt from GIRM, there's clear official endorsement for the principle of having someone direct the singing of the assembly.
For sure, the job can then be done well or badly, and anyone who
drew attention to themselves by flailing their arms round would doubtless be underachieving in the role. But that's not how I go about it, and I've observed a good few others get good results by the discreet approach too.
NorthernTenor wrote:...distracting the congregation from their prayer or reflection before mass by insisting on rehearsing them.
This, I think, you might have the wrong way round. In most churches the cantor leading the assembly in a pre-Mass rehearsal will have to break up the hubbub of community chatter in order to get started, and this may conjure an atmosphere of (inchoate) readiness for the celebration to follow, where there was none before.