Agree with the others. IMHO, people who can't really sing have a place in a choir, but not in a P&W team. The P&W team is a small unit and everyone has to carry their weight. The team has a responsibility towards the whole church - off-key singing can be a hinderance towards the whole assembly: people will pay attention to the singers and not the worship. As far as I'm concerned, that's a no-no (the same applies to musicians, btw).
In a choir, instead, even singers who aren't great can be used to make up the volume. Plus, if the choir is big enough, they'll just have to follow their section (alto, soprano, tenor) leaders (ie, people with better singing skills) and they'll probably be ok.
Having said that, deliverance for the P&W singers themselves can come through their ministry - so it's the MoM's responsibility to give them all the help to improve their skills and maintain the position.....