A choir that everyone can be part of, that values all contributions, and where everyone can have fun and learn is fantastic, and I think every primary school should give children the opportunity to sing together for fun (and not just bloody "Who Put The Colours In The Rainbow" at 9am, with all of those boys who've decided that being in tune is uncool droning along in a continuous B flat — does that still happen?).
But I think maybe choirs which develop skills to a higher level, tackle more complicated pieces, produce an enjoyable, musical sound, and can compete in primary school choir competitions, might also be something kids should get an opportunity to try. It would be dispiriting for those whose aptitude lies in this area, and who want to be part of a group that's working towards high quality performance or participation in competitions, if they can't ever have a go at the more complex or demanding pieces because the music needs to be accessible to everyone, or if every time they go to a competition they come dead last because a few members struggle and affect the performance.
It can also affect how the more advanced members of the choir develop and perform — partly because more focus is on developing the skills of the members who struggle more, because that more drastically affects the sound, but partly because in a choir the members — especially within a section — kind of lock onto each other, get in sync with each other, and keep each other on track, which is especially important for those who, like me, maybe aren't the very best singers, but who can manage when around singers of a similar skill level or better.
I'm only a very mediocre choir singer, but was part of a competition school choir at primary, an all-comers school choir at secondary (though by that age, the ones who really can't manage to approximate the right notes have usually self-selected out), and a fun choir as an adult.
I left the adult choir, because another alto who stood next to me was unfortunately completely tone-deaf and utterly unaware of it. I'm not a good enough singer to be able to stay locked to the notes I'm meant to be singing as part of a small alto section (which is almost always doing harmony stuff, meaning note sequences aren't very intuitive) in a choir doing 4 or 5 part harmony, when someone is very loudly singing entirely wrong notes a foot to my left.
I left, making excuses about why, because it was a fun choir, everybody was welcome, and it was entirely down to me to deal with my problem with staying on-track. I could no longer solidly key into my section, felt I wasn't performing well, and I didn't enjoy it any more. In an inclusive choir, that's my problem, but if you want to run a choir, you'd be aware of how a really struggling singer can pull down the weaker singers around them. It's much more noticeable to me when performing as part of a choir than when performing as part of an instrumental ensemble, too. There's something special about singing as a group that pulls you together into a sort of composite organism — I think that's why religious and cultural groups like to use it, to help form a group identity. (I'd also want to leave a choir where I was consciously aware that I was lagging a way behind in ability, affecting the overall sound with my own voice and making it more difficult for other singers around me, or to be told by the leader if I wasn't aware.) It's not quite like mixed ability with many other activities.
So if a school really can't run both… that's a tricky one, for me. If they do assemblies there might still be group singing for everyone, but I don't know that it really gives that choir experience, and choir singing can be fun and beneficial for all abilities. But being part of an auditioned choir that's working towards particular musical goals is also valuable for kids whose interests and abilities lie there, who might not be great at sports or academics, and whose parents might not be able to support them being in an external choir.