On the Porch of the Maidens, the columns (carytids) have been taken to the museum in Athens because otherwise they were becoming dust. Some of the details have been lost, as you can see in this image. I believe the maiden on the corner may still be original, and you can see how her face and the drapery are wearing away. The figure in the back right is a copy and is not nearly as subtly done (as can be seen in the back of her drapery).
Yes, the Greek protected these works, but they waited until nearly 200 years after Lord Elgin "took" the other works to England. The acid rain in Athens is eating the works that do remain.
Yes the maidens in the museum were degraded and are being preserved by the Greeks. The Caryatids were moved to the museum in the 70's. I had the impression none are still left to the elements. As they are, they are still beautiful. They are not dust and at this point preservation is no excuse for theft.
As an aside, the British Museums treatment of its Egyptian artifacts is abominable. Priceless artifacts are left out for countless museum goers to rub their grubby fingers over them. I say send them back.