I can't really see the rationale for limiting .nz domains to New Zealanders, could you explain that further?
With regard to "useless purchases", that is really a matter solely for the owner of the domain to decide. After all, who gets to decide what kind of use is good enough?
The point useless is the domain not being used, in other words squatting. chandler.co.nz is a example. Ever since that has been registered it has not been used.
The Idea is similar to what the aussie have, and to protect users from pishing sites. (yes there was one, buymore.co.nz).
This would make it harder for scams to set up shop.
I've got a domain I've had offline for over a year while I setup the thing. It's an issue of how much I want to say about what, and why I'm not mentioning more is something else altogether.