One of the most important people at a networking event is the organiser!
This person knows who is coming and what business they are in. It is their job to make sure that your experience at the event is a good one. However, if you don't attend there is no way they can introduce you to other businesses.
Also, please remember if you have booked on you could be taking the place of someone else who wanted to go. This happens a lot at the events I organise. We are fully booked and there are people who would have come though on the night a number of business owners don't show up. What does that say about them as business owner and their companies?
By not turning up can do more damage to a business than attending. If they have let one business owner down will they let a client down?
Of course there is also the motto "you have to be in it to win it"