That works great until something in the stack or system config isn't quite right. Moncef's script handles a bunch of the common failure states. I posted it because it saved a colleague a ton of time un-borking a homebrew ruby install earlier today.
And the main reason to use the script is to save time, which is our most valuable and limited resource. The Prime version installs a lot more than just Ruby. You can set up a new Mac with all your favorite dev tools and Mac apps.
Imagine a growing company that builds Rails apps. What would be more efficient? To have every new engineer set up their machine manually on their own, or have them run a script that installs everything they would need?