I think people who insist on singing through a concert are making a statement, 'look at me I'm a proper fan, I know all the songs'
Some years ago Martin Carthy was booked at the Everyman folk club Liverpool.He was joined all the way through his first song by a man in the audience. At the end of the song Carthy looked puzzled, laughed and said 'did someone sing that with me' Thankfully we never heard from the man again.
When I go to hear a live concert, I know the music is seldom the exact same as the recordings. It's interesting to see what the artist might do with the songs and music, without the distraction of someone who knows the CD inside out.