I don't agree with relying on the many eyeballs argument for security, but from a privacy standpoint, I do think at least the availability of source to MY eyeballs, as well as the ability to modify, recompile, and deploy it, is better than "trust me bro I'm your uncle Steve Jobs and I know more about you than you but I'm a good guy".
If you want to, for example, compile a GPS-free version of Android that appears like it has GPS but in reality just sends fake coordinates to keep apps happy thinking they got actual permissions, it's fairly straightforward to make this edit, and you own the hardware so it's within your rights to do this.
Open-source is only part of it; in terms of privacy, being able to see what all is being sent in/out of my device is is arguably more important than open source. Closed source would be fine if they allowed me to easily inject my own root certificate for this purpose. If they aren't willing to do that, including a 1-click replacement of the certificates in various third-party, certificate-pinning apps that are themselves potential privacy risks, it's a fairly easy modification to any open source system.
A screen on my wall that flashes every JSON that gets sent out of hardware that I own should be my right.