If you want to go back to Kant (my grasp is a bit shaky but it will do), the reason we care about conscious beings over non-conscious beings/things is because they are the source of the "good will" and produce value in the world. The world is dark and valueless without thinking being to grant value to the valueless. If dolphins are conscious, then they too are sources of the "good will" and are deserving of treatment befitting of it.
However, Kant's philosophy leads to some strange places if you push it too far (like all philosophies). I think humans innately combine a mix of Utilitarianism with Kantianism to come up with intuitive judgements - which is a pretty obvious and useless statement, but if we take it at face value it means that when we are confronted with extrema, we would do well to consider both sides of the coin.
However, Kant's philosophy leads to some strange places if you push it too far (like all philosophies). I think humans innately combine a mix of Utilitarianism with Kantianism to come up with intuitive judgements - which is a pretty obvious and useless statement, but if we take it at face value it means that when we are confronted with extrema, we would do well to consider both sides of the coin.