At least seen in the Chinese American communities.
When singing tough musics, the notion of "it's the responsibility" of those who could sing better to "take care" or "guide" the others is utterly WRONG. This promote multiple leaderships, chaos - although, this side-effect is not usually obvious which is why many love to do it.
However it is important for those who have trouble singing to find a way to keep up. And if the whole focus is on this, I'm sure their will definitely be "taken care of". And the responsible leader would do his/her job about it.
As for conductors, it is not the conductors' job to train those who could not sing. So how? Wise planning is required. That is why many choirs have sectional training.