It's valuable if you care about transmitting. Sure you can transmit under part 15 or other unlicensed transmission rules, but you have to maintain extremely low power that is typically not very useful for communication.
On PO Boxes, I don't think it does much to thwart a person looking for your address. It's one single barrier, but anyone can find anyone's legal mailing address via whitepages/public record using only a name and location.
True, but there are a ton of cool things you can do just in ISM. And depending on where you are, your local repeater may not be the most welcoming place, especially if you're not the typical middle age white male.
Depends what you're looking to do. ISM bands are limited to 1W transmit power, and the lowest frequency band with more than a few kHz available in the US is the 902-928 band. High frequency and low power combined tend to mean relatively short range. That being said, I work with 900MHz radio for a living (among other bands), and you can get impressive results in the 1-20 mile range with the right technology and infrastructure.
A $10 ham license opens up a significant amount of bandwidth, and power up to 1500W.
Also, if you do get a license, register it to a PO box.