Most of the sessions I go to involve 'the rising generation' - most of us are both players and singers. This tends to mean it just depends on what everybody feels like as to which way the evening goes, but more often than not recently it will be mixed.
Most of the sessions I go to also tend to be in quite noisy pubs so generally unaccompanied (solo) singing will just get lost. But we frequently end up making them shut up and listen with multiple harmonies!