I thought about that, but like falcolas said, they are at least signed and have some attention paid to them. At one level, any build/install tool is executing

I thought about that, but like falcolas said, they are at least signed and have some attention paid to them. At one level, any build/install tool is executing extremely complicated code on your machine, and could do lots of weird things. The advantage is that you can learn what it does, and have trust in its authors. So well behaved custom scripts created by a trusted party that has articulated standards about how things should work are much better than using shell scripts from an arbitrary project. Still, I'd certainly prefer it if the distro didn't have custom scripts either. It would simplify the task of knowing "this is how the distro does things, and there are no exceptions".