I think YAML fixtures still have value for testing. For example, I use fixture_builder to generate fixture data in Ruby and dump it to YAML files. You get the best of both worlds: define the data in Ruby (even using FactoryGirl or other test data library), and load from YAML files for fast test setup.
It might make sense to extract the fixture stuff into a gem so it can be more loosely coupled, but I’d expect resistance to removing the feature entirely.