If the point of eco-tourism is for people to be able to see unspoiled places, it seems to me this is a good thing for many of those places (certainly not all of them). There is no guaranteeing that such places will not end up like Benidorm in the long run, even if there is no eco-tourism. If such places are places of beauty, what's going to prevent more large-scale building in them eventually anyway? But with eco-tourism at least, the owners of facilities in such places have an incentive to keep the surrounding areas unspoiled rather than building them up, or they will lose their clientele.