One Camera or Two
A number of phones now incorporate dual cameras, including devices from HTC, LG, Huawei and Apple. Is it worth it?
One implementation of a dual camera phone is that one camera works in black & white and the other in color. The principle is that the black & white camera can capture more accurate texture and detail, with the other camera capturing all color information. So each final image is a combination of the photos taken by both cameras.
Another implementation is to have different focal lengths for each camera, allowing a wider or closer image without the degradation of quality that comes with using a digital zoom. The most appropriate camera is chosen for each image.
Two cameras can also provide a certain amount of depth information since there is a distance (even if very small) between the two lenses. This can be used to create depth-of-field effects that mimic larger dedicated cameras.
So, in theory, yes, it is worth it. But it's far from the only factor. The manufacturer may have compromised on the quality of each camera to control costs, whether that's with the lens itself or the sensor behind it. In which case it's entirely possible that a single camera could still deliver higher quality images.
Look for reviews and example images and don't assume that dual-camera is necessarily better.