Part of the reason was for glamour purposes, before the actual glamour system was added, allowing two different style rings to be equipped (it's a lame excuse I know, but there you are). As a glam enthusiast who has different rings glamoured (my wedding ring on my left hand and the Meteor Survivor ring on the other) I can appreciate the sentiment, but yeah, it's frustrating stat wise trying to build up BiS gear and having to equip a slightly lesser ring due to the 'Unique-Untradable' status of tomestone/endgame 'blue' gear.

SE have heard the complaints though and have been slowly changing older blue/tomestone rings by removing the 'unique' status for them, however this is a slow process and the current endgame/blue rings (Moonward, Radiant, Asphodelos etc) still have the 'unique' status and so you can't equip two of the one type. It's likely they'll get to the current ones eventually, but, we'll see.