Two disadvantages:
1. It increases memory consumption, in usual case many (if not most) strings from standard.catalog will be unused.
2. Complicates code a bit, as application will open two catalogs instead of one (plus error handling). The programmer should also make a distinction between strings coming from application catalog and standard.catalog.
Considering that in my opinion only 5 to 10 percent of strings in an app are standard, and also most of languages have some established dictionaries of these standard expressions, there is little gain in such catalog.
