You should really only book if you're interested in the dessert and drinks. BTW, there are plenty of stands and kiosks that sell specially themed drinks out in the park, and most restaurants also have themed desserts for sale as well.
We did the Lights for the first time last year and I would have been unhappy in the dessert party area. The Lights are up on several streets and while the party area is at the end of the main street where the lights are, there are side streets that would be impossible to see from the party area. Also, the party area gives you a wide view of the lights, but you can't see all the details from it's rather remote location. We had the most fun walking around and looking at everything.
I know it's hard to understand until you've seen them...but you really, really have to be right under the Lights to get the full experience, IMHO.