You should save them - if for no other reason than to swap them with FT'ers in other countries. Although they are near-worthless in the U.S., there are many parts of the world where they come in quite handy. Some markets don't use "Rack Rate" they way we do here (e.g., the rate they'd charge if the Olympics were held in the hotel parking lot). So 50% off Rack can actually equate to the lowest available rate.

Check out the SPG board for a wide range of SPG50 threads. I seem to recall that they work well in Australia, among other places. And in the U.S., they seem to have value on high-demand weeks in Manhattan, assuming you are willing to pay $300 or more a night for a hotel after applying the SPG50.