The problem, as I see it, with using the synthetic propers is that they might well turn people off to the idea of propers before they can hear the really good ones. In other words, they might well react, “If _this_ is what propers are, we don’t want anything to do with them.” While liturgical music should obviously serve the liturgy, the music itself must be compelling; it must have a way of drawing us in. I don’t see that in any of the synthetic propers, with only two exceptions that I would use only with my back against the wall.

When I was in a parish that still used the four hymn sandwich, I introduced some propers very slowly–but they always came from the Roman Gradual. I think a good program of gradual (no pun intended) introduction is to start with the Communions, as these are the simplest of the chants (usually), and it’s also a time in the liturgy where a lack of congregational singing would not be so shocking.

Of course, at this point, I have not devoted much thought to gradualism because I haven’t the patience to take a job in which it would be required.

As far as synthetic versus authentic propers go, I would tend to use the former in a continuum if I were in charge of the music for a parish that had a well-formed liturgical vision; but always reserving the authentic propers for the principal Mass of the day (regardless of whether it was ordinary or extraordinary), and always educating people why they are used there.

Lastly, where would the polyphonic propers written by Byrd, Palestrina, et al. fall in this definition of synthetic?