Quote Originally Posted by wereotter View Post
My guess would be that they want to set caps on how much of a sub stat players can actually get (though this is just speculation)

If the rings were made to be not unique, then you could run into situations where you could get a job that gets just enough extra substat to break into another tier, making it much more powerful than they intended it to be in raid scenarios. For example, the Omega ring this expansion isn't great for monks compared to the tomestone ring. If a monk could get an additional 150 points of critical hit from stacking two ilv 400 tomestone rings, it might give the monk more critical hit than was intended for the job to obtain at max gear level this expansion (I haven't done the math to prove or disprove this specific case, just using it as an example)

Additionally, items might be tagged as unique to help idiot proof the tomestone gear system. If you can't buy another item you already own, then you're less likely to make a mistake when purchasing gear. This is less applicable when it comes to half your gear being upgraded, it it is possibly something they considered in making the items unique.
That sounds like they would need to split them into Left and Right rings and give them different stats if what you say is actually the case. Which very well could be. It could also be as someone said SE trying to get people into other content... But I don't feel like one ring is going to make people do Savage raiding that are not already doing it for the weapon / armor / gear.