If you normally sing unaccompanied by instruments it can be very offputting to have someone suddenly join in uninvited. When you sing without instruments you are free to adjust the phrasing, timing, tone , you can pitch it to suit the song and voice and even vary this within the song as needed to convey emphasis and meaning. I make the distinction of singing unaccompanied by instruments because in many singarounds singers tend to choose mainly songs with a chorus or refrain to enable others to join them for that part of the song. Chosing to sing without instruments is not precious but it is a different type of singing, just as there are different styles of playing instruments. In the end it all comes down to freedom, appreciation, consideration and most of all manners. IMHO
JudeBTW thanks Deni