This is how things used to be done in the old days, for example on SunOS 4.x. At least for any software you added to the system, where you'd add a line to /etc/rc.local.

It was simple, but it was also error-prone because you're re-inventing the logic every time. And it made it really difficult to automatically install software because instead of dropping a file into a directory, you had to programmatically edit a common file. Installer scripts would do all kind of horrible things like "cat myscript >> /etc/rc.local". The situation forced software maintainers into doing something (automated editing of code) that they weren't skilled at, with tools (Unix commands) that weren't very well suited for it.

Having a more data-driven, standardized approach split up into multiple files was a huge improvement from a maintainability standpoint. Yes, it's more complicated, but in my opinion the complexity was easily worth it.