This feedback is really good and really useful!
Team Sieni's research suggesting a UK and an International association, and Silver Quill's info about Canadian UU churches suggests to me that concerns over geographic limits are easily overcome.
For my part, I only seek to add subcategories because the more generic church categories don't work very well in my area. "This Old Church" category is fine if it's 100 years old, but I came across a church, built by Louis I. Kahn in 1962, that I waymarked under subcategories of "Architecture" ("Master Architects - International Style " and "Great Buildings of the World"), but couldn't waymark as a church per se. I'm not saying this particular church needs cross-marked again, but why shouldn't such a fine church be represented in the religious buildings category? I don't know all the ways waymarkers use this database, but if I wanted to visit a church in my travels outside my home church, I would look up churches on Waymarking.com at my destination. The most generic of church categories doesn't cover enough ground to serve as a "catch-all", nor should they ... right?
And what about interfaith chapels? At present, they can't be waymarked anywhere, again, because there is no catch-all subcategory. Some interfaith chapels are among the finest architecture out there. I'm thinking of the chapel at the Air Force Academy in Colorado for example.