As a long-time half cyclist, I got an earful on the subject by insiders who live, eat, and breathe tandeming. So let's get the misconceptions out of the way.
Tandems are nothing more than a novelty and offer a lesser cycling experience.
Tandeming is all about enjoying teamwork and goals with another rider. For riders of unequal abilities, tandems can bridge the differences and provide a shared athletic experience. Tandems are slow, especially on hills. Tandems with two strong riders can outdo the athletic feats of any solo cyclist. Tandem teams have double the engine power at half the wind and rolling resistance, big speed benefits.