@jedp, @ypwalter , at that time (and still now) partners are using this test suite to run tests on their own builds. I wanted to keep the tests open to run on any kind of build with basic Gaia and marionette enabled and not be restricted to mozilla builds that have this engineering-only build.  
If these tests use UI-tests but don't block any other test functionality it's ok I think. The example @ypwalter mentioned was for keyboard tests - which is not really a test that should be specific to engineering builds.