Linux being open is not just about the kernel source code. It's the fact that it's modular, that you can decide what brick you need, add or remove them, and end up with the perfect right configuration.
It's especially interesting when you're building an embedded system (typically with a RPi). Start with the minimal Arch Linux install, think about what you need and add just that.
There is a lot to learn about how operating system work when you configure Linux without having to recompile your kernel (something that nobody do any longer anyway).
Or you can go even further, use something like buildroot (http://buildroot.net/) to build your kernel and rootfs, and even tweak the kernel configuration if you like. I recently did an RPi project where the kernel and rootfs together were about 5 MB compressed.
It's especially interesting when you're building an embedded system (typically with a RPi). Start with the minimal Arch Linux install, think about what you need and add just that.
There is a lot to learn about how operating system work when you configure Linux without having to recompile your kernel (something that nobody do any longer anyway).