We have a small 10-12 person "Mass" Choir that anyone can join - so we have a Monotone section. This has been a test on my patience - usually am the type who wants to pinch or punch out people who sing flat, etc. But I can see the souls behind the rough voices - God must be working on me about that.
The weird thing is that in the months we have been in existence people are STARTING to sing more on pitch (Not quite there yet, but better). Most people can sing on pitch in their range, but get off if they go too high or low. Putting the weaker singers by the strong ones helps. Our "Monotone guys" are actually Basses. This is a problem in a 3-part SAT Gospel choir, so I usually put them by the sopranos and tell them to sing the melody.
Last Sunday we did a song with a built-in Bass part - "All My Help". I panicked a bit when I was told that our strongest Baritone/Tenor was out, and the other lady Tenor didn't show up. But Pastor said the God is in control, and the Basses carried the Men's part off - not perfect, but they could do it because it was right in their range. Where they really surprised me was when we did a medley of Xmas carols. They were able to sing them mostly on pitch, because they were familiar with them...