I book in advance sometimes but I do not like the way RCI does it. Seems to me the way Princess does it makes more sense, less of a problem in case things change.
I booked in advance for my cruise in Sept. I think it's kind of a hook that they say limited availability but they got me anyway! :-) Reading about Vigo, Spain not much there so I decided to do a tour.
In the Caribbean, I book ahead of time for the snorkeling/catamaran or anything of an active nature, 4 wheelin', rafting, etc. because I do think those sell out!