It looks from the comments so far as if most people prefer to hear the occasional song in a set.That's how I feel myself, and also what I've noticed in informal pub sessions - the non-participating public often ask for songs and show more interest when there's singing.

But I think it would be a mistake for a musical duo to sing just because they think it's required. Not-very-good or half-hearted singing is probably worse than none at all, and might just undermine your credibility as musicians. It would be better, I'd say, to invite a singer to join you and do a couple of numbers in each set (assuming you're doing club-length sets of, say, 40 mins). There are plenty of good singers who'd jump at the chance of singing with a competent accompaniment (Well, there's me, for one...). If you're just talking about short floor-spots, instrumental music on its own should be fine, as the MC can introduce some singing before or after.

Marje