It's obviously not really possible to tell how effective they are, but it seems to me that if a buyer has found one of your images to be worth viewing, then it can't hurt to put any similar images or a related lightbox link into the description - if the one they found isn't quite right, they are quite likely to look at the others first before searching elsewhere, and you get the sale.
I've several private lightboxes with links from images, and they get hits, some in the hundreds - so obviously people do look.
UBB links are harder to judge, but as you would probably only put a few similar images in as UBB links they could be even more likely to be of interest and get sales.
Besides, it's not difficult to do, so why not? I always feel it's giving me a little extra influence on the marketing too, something I can do for myself to help my image exposure rather then just relying on the site search and the best match.