We are hired primarily as an instrumental group. We can sing but it seems that most of our clients are looking for just music during their ceremony. Sometimes we get a request to accompany a singer that is a "friend". I know a lot of musicians that specifically do not want to do this. There are valid reasons for not wanting to do it. It is more work for the musician and it introduces an unknown element into the day.
My thinking on this has changed over the years. For me it boils down to what the client wants. Presumably they know what to expect from their friend or they wouldn't ask them to be in the service. Even if the performance is less than perfect the important thing is that the client is getting what they wanted on their day. BTW I can just about guarantee that any live performance is going to be less than perfect!
The one thing that we insist on when a client introduces a singer or another musician into the mix is a rehearsal session. This gives everyone the best chance to deliver a quality product on the day of the wedding. It also gives us a chance to meet each other and hopefully feel more comfortable with each other on the big day.
This year we have had the opportunity to work with several really great singers. In a recent wedding a lady from the bridal party was going to sing the song " 1000 years". She was from out of state and could not meet with us until the day before the wedding. We did our homework up front to make sure that our arrangements matched. The bride to be and the entire bridal party came to our home for the practice session. After the first run through we knew this was going to be a special performance.
When we were finished with that song the bride to be and two of the brides maids started singing an acapalla version of a hymn. The three part harmony was tight and the sound was amazing. Turns out they were in a vocal group together in college. After we heard the level of musicianship that the client had, we were very happy that she had selected us to be her wedding musicians. Here's a video of the performance we did with Mavis Bortey-Fio.
Now there was that one time when we had a singer that sang the entire song in a different key than we had rehearsed....but that is another story!