Because they only have 10 species, it is veery expensive for what you get and yes they have rarities, but Blijdorp, Burgers Zoo and Antwerpen have much more rarities.
It's more a themed atraction park then a zoo.
Lintworm, yes, You are right, the Dolfinarium is an amusement park as well (the owner is Compagny des Alpes since 2002). But, beside the fact the institution is member of the International Association of Amusement Parks and Attractions, they are also member of the Nederlandse Vereniging van Dierentuinen, European Association of Zoos and Aquaria, International Association for Aquatic Animal Medicine, International Marine Animal Trainers Association, European Association for Aquatic Mammals and the Alliance of Marine Mammal Parks and Aquariums. Furthermore, taken into account that the institution has a special educational system (especially for children), and such a proper and unique captive environment what they provide e.g. for their Dolphins, I dont think so we can call the Dolfinarium only an amusement attraction, which can be spared the interest of zoo professionals and professional zoogoers. Of course, the number of the exhibited species is low, but dont worry, many zoochatters tend to visit collections only for the sake of one species

. The difference of the admission fees between the mentioned big Dutch zoos and the Dolfinarium is not so big in my opinion, its worth to pay eight more Euro to see a marine mammal park than a ’typical zoo’. Of course, everyone has the own reasons visiting different zoos and why visiting the given collections. When I was in the Netherlands, my aim was not only to see the bigger zoos (Blijdorp, Artis, Arnhem), but visiting specialized animal parks, like the Dolfinarium and Apenheul.
Well, its up to Shirokuma, but shall continue to give any suggestions, Dolfinarium Harderwijk is absolutely worth a visit, because this institution is very unique in Europe from many aspect.