There's no difference between form factors (we play these sounds in desktop Plasma too), the difference is in the events. When I plug in my laptop, having a confirmation that it's actually charging is useful, because if it doesn't play, I almost certainly want to take action and fix whatever problem prevents it from charging. Similarly, when the sound for unplugging the charger plays when I don't expect it to, I almost certainly want to figure out why it stopped charging and fix it.
In contrast, most people in most situations don't care if a device is fully charged, it just stays connected to the charger and that's it.