Observing through a telescope with just one eye is actually unnatural. Foregoing one eye reduces the perception of detail and is also uncomfortable. Continual relaxed observation is only possible with both eyes.
A binocular head is particularly worthwhile for bright objects. The objects appear more three-dimensional, and full of detail. With the Moon, you get a feeling of floating over the surface.
Binocular heads use beam splitters and prisms. The resulting loss of light can be too great for faint objects. In addition, the binocular heads require a long light path and represent an additional weight for the focuser.