I can't speak from any personal experience, but I myself am going to be going to Plitvice in June. We opted to stay one night there simply because we've heard that although its straightforward what you'll see there the park is big and it is enjoyable to explore. If you want to explore the whole park, you will definitely be rushed getting in before noon and having to leave around dinner, a night gives you flexibility and time to relax and take in the sights. So if it doesn't impact your itinerary that much, I would say spending a night there is worth while just to make things easier, especially since hotels there are very cheap.