I much prefer a good mixed session - the problem is often getting a good balance. There is also the basic difference that unlike music sessions where everybody jams in together, singers will often prefer to perform on their own. This is not because they're all prima donnas, but because the timing, delivery, and phrasing of a song are very personal things and unless someone playing an accompaniment knows exactly what the singer wants, they can completely spoil what they are trying to achieve. Obviously on certain well-known sing-along songs, everyone is welcome to join in, but I once had to shut up an electric bass guitarist tryin to back Essex Girl on a gentle unnacompanied song. On the other hand, Leadfingers is welcome to add one of his beautiful whistle parts on most of the songs which I sing - but I'm not so sure about the banjo :-)