One of the key questions this branch of science addresses is whether life requires an ingredient which can not be artificially reproduced (however vaguely defined as a soul or divine intervention). If a robot, or a virtual pet, can behave eerily simular to a natural equivalent, the argument of the mystical "breath of life" finds itself on shaky ground.
Designing "human-like" robots, although they are nowhere near of having the intelligence of a human, can help us to study ourselves. Imagine a robot that is able to express the direct opposite of the mood you are in. If such a robot is built, it implies that we understand which hints a computer should take from a human face to infer a mood and how to represent a mood in an artificially created face. Aspiring to recreate emotional expressions, requires that we have an understanding of the mechanisms behind our own.
Having said that, Honda's robots which can walk in a human fashion, or Sony's doglike Aibos, are indeed more of a marketing trick than in depth research in AI. They reap the media attention a commercial enterprise can benefit from. Yet, steady advancements in these technologies can benefit us not only in terms of entertainment but hopefully also when it comes to replacing our defective or ill-stricken limbs and senses with artificial ones.The more AI resembles us, the more we can use it to augment ourselves, when necessary.