Yes it replaces the role system a well. Simply create a new role entry in your database table.
With that said you CAN still use the old membership system. The new one lacks many features and it meant to be as the name says ... Simple.
The web forms templates for 4.5 don't use simple membership at all so it's up to you.
I prefer simple membership as you deal with the tables directly for the most part. Simple membership has also been around for a couple years as part of webmatrix which is where you'll find the references point to.