There is currently no released version of Android that uses ART out of the box. It's available behind the development options in KitKat and as default in the L developer preview, but that's it. The "optimizing applications" screen comes after system upgrades also with Dalvik.
I think it took root in part because even end-users who were only casually interested saw a dialog suggesting as much on upgrade. When you upgraded to an Android version that used ART, you got a message telling you that the apps were being precompiled, which took a while to finish.