Agreed. I'm hypothesizing that you might do this, even with keys intended to be used as a back door, by shipping it on devices, you vastly increase the number

Agreed. I'm hypothesizing that you might do this, even with keys intended to be used as a back door, by shipping it on devices, you vastly increase the number of potential suspects for any backdoor abuse. Continuing this train of thought - a hardcoded password is classic example of a backdoor, and just as "public" as including a private key.