The best perk is that ability to rebook while on board at 10% savings plus a $200 per stateroom onboard credit.
In the distant past, you had a grace period during which time you could rebook, but the deal wasn't as good.
And yes, one "frequent flyer" gift per stateroom and the reception. On most cruises 1/3 to 1/2 of the cruisers are repeaters. We're 10 down on the Magic, with 11 and 12 booked.