I think you may be confusing the Priority Embarkation available to upper level VIFP (Carnival's past guest program), which is free to them, with the Faster to the Fun Program, which you much purchase. You can get more information about that here;
Frequently Asked Questions, Information | FAQs | Carnival Cruise Lines
Whether it's worth it or not depends on whether you want to spend the extra $49.95 per stateroom to purchase the benefits.
Some people feel it's worth it, while others don't. Just a personal choice.
In my opinion, I wouldn't pay the extra money - not worth it to me, but that's just my feeling. It's just another way for Carnival to take your money. But it may very well be worth it to you, especially if you're traveling with small children.
Pete