I used to use the retainer method last tier because of having an extra Alexander STR ring, one with VIT meld for tank, and another with a Crit meld for the DPS jobs. Most of the time I used the VIT one on tank and DPS just because of traps.

Although some people here just want the easy way so they can equip two tome rings. Which you can, just one has to be upgraded to 340, and that requires a glaze from O2S. Or just get a Genji 340 ring from O1S. Otherwise why does it matter if you aren't doing high-end content where higher ilvl matters.

The unique restriction is just current inventory, also prevents needing an item again, or accidentally messing up pages, or a weekly tome purchase.