"If you are outdoors, you are or should be aware of the weather.
The only time a siren is useful is if you are inside and not aware of the weather outside."
I disagree. Sirens are not meant to be heard indoors. If you can hear them inside, awesome! But that is not their intention. They are to warn people outdoors. Although I can hear the sirens in my house, there was a night I slept with my windows open and the sirens went off, and i did not wake. (My case I need to buy a weather radio.) It can be a sunny day and a storm can pop up out of nowhere. Having the mixed system in place to warn the public will aid in saving lives. Not everyone takes those precautionary measures seriously, but at least all has been done to warn the public of threatening weather.