Ok, so we debated this feature at length and our reasoning is as follows:
a). We wanted to be able to show really big, oversized images of the thumbnails being clicked on.
This meant buying some screen real-estate which was far too large to just push a few bits aside on the main product page.
b). We wanted to be able to showcase the image being clicked on without the distractions imposed by all the other features on the product page. We still feel that the simplicity if the lightbox lends to a far cleaner user experinece and really shows of the piece being viewed.
c). To bring in a retail concept, the light-box was intended as a way to get close and intimate with the product. Short of being able to touch and feel the product we hoped that an oversized light-box will bring the customer that much closer to the product they have chosen to examine more closely.
Ultimately the points above lead to better sales conversions and greater customer satisfaction.