silhouetting can be a beautiful effect and hence very desirable, like in the above photo. it is easily achievable in certain light conditions, but in some circumstances, we could choose to avoid it.
two more examples where the effect could give a very beautiful result.
In case we choose to avoid it, and given that our subject is within reach of our flash, we can artificially light it and get a very different result (note to the position of the sun in this photo).