I don't think it's about giving full allegiance to one or the other. It's just about a person's individual preferences.
Since I don't really enjoy roller coasters on their own, I'm probably not going to ride one if there isn't elaborate theming (and still may not depending on how intense it is). I'm really only interested in elaborately themed rides, which is why I really only care about Disney -- and Universal to a slightly lesser extent, since they have a weird mishmash of themed and unthemed; certain areas are great and others offer almost nothing to me. I'm really only interested in theme parks. So something like the Jurassic Park coaster going in at Disney would be a huge detriment to my enjoyment, since it's not a ride I'm going to get on and it detracts from the theming of the surrounding areas. That doesn't mean it's wrong to build a coaster like that, or that people shouldn't be very excited about it -- it's just that it's a net negative for me personally.
Of course, I also don't visit parks regularly like others do. I go to Disney and/or Universal once every few years, and go on non-park related trips for the most part.