A classmate of mine (who works at a very high, professional level,) recently made the comment in class about, "we don't sing something during Mass just for the sake of singing it."
She gave some sort of example like how we shouldn't have the choir sing "Sheep May Safely Graze" just because we want to learn it, but instead should just sing Easter pieces at Easter, etc. (I can't remember what the exact examples that she used were.)
I've been thinking about that assertion quite a bit since then, because, honestly, that's something that I do all the TIME! I find a bunch of pieces that the choir could learn and that look good, and then I kind of have an idea of what order they'll go in, occasionally I'll find one that fits a particular Sunday particularly well, and then they'll sing it when it sounds good (which I usually have a pretty good idea about when that will be.)
Anyhow, a particularly limiting factor for me is simply due to the skill level and numbers of my choir. It is VERY difficult to sing anything in 4 parts, it takes quite a few weeks so I can't just pull out any old 4-part anthem and be like, "oh, we'll sing this on this week!" So I sift through the internet looking for quality 2 or 3-part pieces, or easier 4-part, and since those are all somewhat hard to come by, and I try to do one choir piece every week, well, if it works then we'll do it!
Then, there is the whole other topic of how to "pick" pieces for Mass. Going on the assumption which I have written about before that the Propers are the primary choice of text, and *not* that of the "theme" of the Mass for the day (which may or may not exist,) that brings a whole new level to the debate. What can be sung *after* the Proper is sung? Can't anything? At St. P, the congregation sings a simple arrangement of the Introit in English, a very simplified arrangement of the Offertory after a hymn has been sung, and a small schola of women sings the latin Communion chant. Once that has been sung, then can't *any*thing be sung? Therein lies the dispute. Some people firmly hold to the belief that every Mass has a "theme," however, I subscribe to the belief that (especially since V-II rearranged all the readings,) most Masses are just a hodge-podge of scripture readings, and certainly don't relate to the "theme."
Finally...what is the "theme" of every single Mass?
Repentence, Praise, Adoration.
In that order.
I'm pretty sure that any piece that I have the choir sing will fall into one of those categories...