My big thing with exclusivity is that a lack of it can lead to an idea of USH as just "UOR but smaller."
Let's say there's a hypothetical Nintendo fan somewhere in the middle of the US and they need to make a decision about which park they're going to visit. If both parks have Mario Kart and Donkey Kong, then they're almost definitely going to choose Orlando because Orlando also has the Yoshi ride, which Hollywood doesn't, and it's a larger resort outside of the Nintendo stuff. But if UOR has Donkey Kong and USH has, say, a Luigi's Mansion dark ride, then it makes it a more complicated decision. It gives people an additional reason to visit USH and it makes USH a destination rather than just UOR with less stuff.
We saw that play out with Potter when it opened. It boosted attendance and revenue, sure, but not nearly by the numbers they were hoping. Hogsmeade in Orlando had been opened for six years by that point and boasted an additional thrill ride and Orlando also offered Diagon Alley. There was nothing you couldn't do in Hollywood that you couldn't also do in Orlando but there was so much you could do in Orlando that you couldn't in Hollywood so, unless you lived in the immediate area, why would a Harry Potter fan choose to go to Hollywood?
That's the argument for me. I want USH to become a destination in its own right rather than just living in Orlando's shadow. Unique offerings give extra incentive to visit USH over UOR.