It frankly bugged the carp out of me when I got the wheels second hand (I'm an enduro racer on a budget) after spending a couple of years building up a good range of gearing (DA and Sugino rings, DA sprockets) only to find out the wheels had a proprietary spline system.
I normally resist changes that make things more 'consumable', ie they wear out faster, or you need to buy a new set of something, but to Easton's credit they have done a very good job of designing the system and manufacturing the sprocket. And they have very kindly got back to me and will ship to New Zealand, which is good of them.
Lack of campatability between these and my training wheels is my biggest issue.
As an aside, single speed 3/32 MTB sprockets will also fit, due to the hyperglide spline pattern Easton uses. Not all proprietary systems use hyperglide (White Ind use their own, as do BMX manufacturers)