Actually it does. The binaries are all included in the .app, the secondary installer just places binaries in the various locations around the HD.

Look at man xcrun and man xcode-select. Both of which can be used to switch to different Xcode versions easily. In the future it looks like they are planning on making it easier to switch between different versions of Xcode.

And using xcrun one could set up a build environment for make/config with CXX/CC/and all other environment variables pointing to the right location in your /Applications/Xcode.app/ folder.

One thing I don't understand is why Apple didn't just symlink the various binaries into the Xcode app if the Xcode app is installed, instead of having to download another 170MB file.