This years Fireworks event is seemingly very close to selling out (this includes MAP pre-bookings)
page on the Alton Towers website has the following information for Annual Passholders:
All Merlin Annual Passes are valid for Fireworks without restriction, however we strongly advise guests with an annual pass to pre-book online to guarantee entry. Even with pre-booked tickets, we advise you to arrive before 3pm to avoid any disappointment. Annual Pass tickets can be booked in advance for £5.
What is interesting is that they are advising people with pre-booked tickets to arrive before 3pm "to avoid disappointment". I presume this means to avoid excessive delays getting into the resort rather than being turned away due to the park being full as this would leave them in very hot water legally.