One of my favourite sources for musical ideas is Choral Net . The other day, there was a question about whether or not a choir director should sing while directing. This is an age old question but as usual I have my own ideas. I honestly feel that there are very few differences between how you work with singers not matter what the age. Older singers often need help with tone placement or intonation as much as younger ones. If you are working with so called professionally trained singers, I think you still have to remind them of basics. Really, all of the following would apply to most situations.
1. Sing to set an example - Often times, it is easier to sing a phrase the way you wish it rather than try to explain it with words. I remember my mum telling me that she had a teacher in high school who never sang. She would get a student to sing examples for her. Mum said that her teacher's inability to sing didn't stop them from being an award winning chorus. So whether it is you or a ringer, sung examples don't hurt.
2. Sing if a part needs help - If we are short on sopranos, I will sing along. If you are a tenor and they need help, by all means sing. Although, it is usually inappropriate for a director to sing during a performance, circumstances alter cases. I agree that if you are singing along all the time, you really cannot hear what is happening with the other parts as well. So the rule is, there is no rule. If you are in a competition, you better not be singing. Otherwise, use discretion and help only if really needed.
3. Be quiet most of the time - With the 2 suggestions above, it seems as if you should sing lots. In fact, if you are singing all the time the rest of the singers begin to depend on you and don't then learn to be independent. This is NOT what you want. You can sing occasionally, emphasis on OCCASIONALLY for specific purposes. Your choir must learn to be independent. You can undo all the work you have done to get people to sing well if you just take over. You don't want to leave your singers feeling overwhelmed but don't always rescue them. Let them find their own way most of the time. That is how we learn most effectively.
So the answer to the question is sometimes we sing to help but most of the time, we do not. You are either the director or a member of the chorus. That doesn't mean that you can't dismount and join the chorus for a sing along time or just for fun once in a while. Because after all, it is about the music AND the fun!!