I suspect the issue with installers being model-specific isn't down to control, but rather down to the precision with which Apple engineers the hardware and software together. Different models of Macs can be remarkably different from one another, and those differences are concealed from the user by the extent to which Apple controls both the hardware and software and can therefore tune the software to the hardware. We don't deal with a lot of the driver issues that are all too common in Windows-land because of how far Apple is willing to go to match hardware and software, and how willing they are to modify OS X to accommodate new hardware.
