The advantage of the indexing that the Rocky Mountain Philatelic Library has done for the Philatelic Union Catalog is that it allows researchers to find Congress Book articles along with articles and books from other sources. So, the researcher does not need to know that his topic has been covered in a Congress Book in order to find it.
If you know that you are looking for a Congress Book article, however, the American Philatelic Congress index may be more efficient.
At the APRL, we add links to online indexes to our journal records. We hope to also work with journal editors to incorporate as many of these indexes as possible into the union catalog. This is especially helpful for researchers who don’t know which journals might cover their topic, and for finding articles in unexpected sources. It also enables the researcher to do a broad search from one location, instead of having to search multiple indexes.