Lets set something straight it's a 'Merlin Annual Pass Preview Day', therefore the day is exclusive to to MAP holders.
It didn't work last time, MAP holders where waiting a very long time to pick up guest tickets and it did add to the final gate figure. If there was 5000 MAP tickets available and everyone brought a friend that's 10,000 guests.
5000 - standard quiet day, everyone has a good time.
10,000 - you piss off all your loyal customers and make them wait excessive amounts of time for rides.