It may have been enabled by default, but only on Windows. On Windows Phone it wasn’t, and I can’t say what the default was for Xbox 360.
We do not currently have an easy way to build content differently between Debug and Release builds. The benefits and drawbacks of compressing content must be evaluated on the target platform. You would not want content compressed on Android because asset files are compressed by default as part of the Android packaging process, though this could be modified (exclude .xnb files from asset compression and use MonoGame’s content compression).