Well, with Dreams Riviera Cancun it might not just be that the bride and groom get a free room. You can earn up to 3 free rooms depending on how many rooms you book through the block. This is given to you as a refund so some brides and grooms have divided up the $$ and given it back to the guests.
Another thing to consider is that a group block is not just for the freebie or amenities. It is so your rooms are blocked so that they are available to your guests. I am sure you have read about brides freaking out 6 months before their wedding because the resort sold out (6 months BEFORE!) and there were no rooms left for their guests.