I am sure people would consider it, you're right. Somehow it seems like a riskier step then getting a vicious dog, given that a glitching defense-bot could shoot you or your family, or your neighbors, package delivery people, etc. Also I am not sure too many people would have the technical skill to build their own, so unless they became a product (with serious, serious liability insurance problems...) it's hard for me to picture them becoming popular somewhere like the US.