Singing with eyes closed is not a matter of confidence, but it does help you concentrate on the song, rather than yourself or the chap in the back that is crunching crips and discussing the footie.

And it's not a matter of those who sing unaccompanied do so because "the poor things" can't play an instrument. More than one person who is primarily a musician has said that for certain songs, even though they could play with them they choose not to.

Lots of people have stated very clearly that having an instrument playing can detract from a song rather than enhance it, and be very restrictive in terms of regulating tempo. This is not to say that some songs cannot benefit from having a regularised beat that doesn't slow down to near dirge when others join in the chorus, but there are many songs where the instrument is unneccessary, a distraction and is there because the performer wants a crutch, or because the musician wants to fiddle with something.

Others may be delighted when people with instruments join their singing uninvited but most of the time I find it restricting. Mostly other voices which join in can adjust to minute alterations of tempo, almost without their owners being aware that they are doing so and thus don't pose the same problem and are welcome as they add to the song. Unfortunately most of the time when an instrument is played the cumulative effects of the minute differences between the timing of how the singer would have sung it and how they sing it because the timing has been regularised often makes a song (which is wonderful unaccompanied) come out very flat and robs it of meaning. There are some very skilled performers who, accompanying themselves can make the transition but very few.

The absolute worst is the musician who constantly noodles along with everything, even when they have NO idea what the song is.