Why is CATKIN_ENABLE_TESTING set to ON by default?

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When running catkin_make on an empty workspace I can see CATKIN_ENABLE_TESTING is ON:

$ mkdir -p /tmp/test_ws/src
$ cd !$
cd /tmp/test_ws/src
$ catkin_init_workspace
Creating symlink "/tmp/test_ws/src/CMakeLists.txt" pointing to "/opt/ros/indigo/share/catkin/cmake/toplevel.cmake"
$ cd ..
$ catkin_make
Base path: /tmp/test_ws
Source space: /tmp/test_ws/src
Build space: /tmp/test_ws/build
Devel space: /tmp/test_ws/devel
Install space: /tmp/test_ws/install
####
#### Running command: "cmake /tmp/test_ws/src -DCATKIN_DEVEL_PREFIX=/tmp/test_ws/devel -DCMAKE_INSTALL_PREFIX=/tmp/test_ws/install -G Unix Makefiles" in "/tmp/test_ws/build"
####
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Using CATKIN_DEVEL_PREFIX: /tmp/test_ws/devel
-- Using CMAKE_PREFIX_PATH: /opt/ros/indigo
-- This workspace overlays: /opt/ros/indigo
-- Found PythonInterp: /usr/bin/python (found version "2.7.6") 
-- Using PYTHON_EXECUTABLE: /usr/bin/python
-- Using Debian Python package layout
-- Using empy: /usr/bin/empy
-- Using CATKIN_ENABLE_TESTING: ON
-- Call enable_testing()
-- Using CATKIN_TEST_RESULTS_DIR: /tmp/test_ws/build/test_results

The default seems to be set here: https://github.com/ros/catkin/blob/5a... What is the reason for this? The example for using that variable includes downloading test data: http://wiki.ros.org/rostest/Writing#W...

Should I set CATKIN_SKIP_TESTING to ON or CATKIN_ENABLE_TESTING to OFF if I don't want to download test data by default?