Why remove "Rails.root/test/mocks/test"

I am upgrading our rails app to rails3 from 2.3.5 and I got the
following deprecation warning: DEPRECATION WARNING: “Rails.root/test/
mocks/test” won’t be added automatically to load paths anymore in
future releases. Question is why ‘mocks’ directory should be removed
in future releases, I think sometimes they are useful especially for
selenium tests.