I agree with GUEST of 9:20 that percussion should be added only at the request of the singer. We have a world-class bones player in our area who is requested to take them out quite regularly, any time he is around. (The only time they come out without asking is at shanty sings when someone singing 'Johnny Come Down to Hilo' gets to the line "I ain't heard nothing but the jawbone since," and at that point people look expectantly at him anyway.) On the other hand, we also have a woman who often has a largish plastic jar of large, hard ibuprofen capsules and 'plays' it; the trouble is, not only is the size larger so the lag time longer, causing the rhythm to be off, it is vastly louder than the wooden eggs.
I wish we had more owners like the one Don Firth describes (10 Jan, 10:52). It would be good if someone were 'officially' expected to pass the word to those who add percussion to the point of distracting others from the song; I doubt anyone would be perturbed over adding them to, say, 'Sloop John B' but definitely an issue with 'The Great Silkie of Shule Skerrie.' A similar issue has been brought up with singing along; it is simply not always appropriate. I have attended a longrunning song session in the west of Ireland a few times, and when someone strikes up a ballad that does not have a chorus to join in, and someone starts joining in, there is a man who says, in a very dignified way, "One voice. One voice." People heed him, and respect it. It is very difficult for the singer to express this directly; it is easy to ask people to join in, but not easy to ask them not to join in until invited.
I have of course seen other instruments join in with a singer and it usually seems to work well; I am not sure how much of this is the nature of a session, of course, and how much relates to differrrent instruments being less intrusive or better handled; I am sure all three enter in. Some sensitivity and courtesy go a long way in making a session work for all; unfortunately these are very hard to teach and quite unrewarding to try to enforce.