Yatse needs to register some events for battery optimization for widgets (Like user is present, ...).
Due to the way Android works it will start Yatse each time those events occurs and it uses way more cpu and battery to start and close an app every time than having it in Android cache doing nothing.
So your Android system will start Yatse regularly it you use Widgets, notifications or Lockscreen controls but since Yatse is optimized if you don't use Widget or Notifications no CPU (and battery) will be used.
Unlike other OS, Android applications does not need to be closed and the OS have complex system to optimize things. If the phone need more resources inactive application will be killed.
Yatse is still ultra optimized and takes care of every bit or CPU and battery it can save. You can check your android battery usage report and you'll see how good Yatse is at this.
You can also read sites like http://androidspin.com/2010/05/25/why-you-dont-need-a-task-killer-app-with-android/ or http://developer.android.com/guide/components/activities.html for explanation of why apps does not need Exit buttons.