If you do the headliners in the morning when you don't need your express pass, you can do them again in the afternoon with the express pass. Or, you can just do the less busy rides in the afternoon.
You can purchase an unlimited express pass also, it is just a lot more expensive.
With express pass, you can ride whatever, whenever. You don't have to come back at a certain time and plan your day around the times of the pass. You can just wander through the parks, using the pass as you get to a ride.
THere are a few rides that don't accept express passes of any kind. Those rides are Forbidden Journey, Pteranadon Flyers, Rip Ride Rockit and the Ollivanders show.
The express pass isn't a rip off. You have to remember that there are many hotels within walking distance of the parks. If the unlimited express pass wasn't a perk for onsite people, there would be no reason for people to stay onsite.
There used to be free express passes offered like fastpasses. However, they ran out so early in the morning, that it just created a lot of frustrated people. They really had to limit these to keep the express pass lines down for those who were staying onsite.