Liches attack with death+fire, in Irazaur tomb you get attacked with death, poison, and frost, in ark you need to protect yourself from fire and shocks. Items with multiple resistances have their uses. Also note, that many of those rings are available early, and/or have other useful bonuses
. Having almighty rings might ruin the balance.
Dude, how silly
Lich attacks are pretty rare and not all of them do mixed damage. But, even if, it is not something ordinary and my aim is not to propose items that protect you from everything at once, rather to offer some opportunity to slim down the items that you keep in the vaults when you have to face specific dungeons.
Of course items that protect you from 2 elements would be nice but ONLY If the bonus they provide is good enough. Usually rings provide the best defense vs. elements making most of the difference, so it is much better to have a +70 vs. death using Greater Ring of Death and Ring of Unlife instead than having a +60 vs. death and +20 vs fire replacing the Ring of Unlife with Ilmaran Ring.
As well, if I use - let's say - 2 rings of Frozen Hearth I have a 18x2 reduction vs. cold and acid, for a total of 36 that is much worst than a total of 60 provided by 2 Greater Rings of Ice OR endurance (acid). Yes, the Ivory ring is a bit better due to the armor bonus and because undeads usually deal death or cold damage, but the Detection II bonus is irrelevant and overall it won't make any difference, while it is better to have a much higher Death protection since cold damage will be minimal in any case.
I personally would prefer to have those rings as Unique items achieved by killing an ancient wivern or gorilla king that provide a +30 bonus vs. 2 elements at once rather than having many of them that are less effective than specialist rings.
Last but not least, I did NOT ask for any almighty ring, because I know that they might ruin the balance, but just end-game unique items to narrow down the list of gear that I may want to keep and with a combo of elements that are hardly found in the same place.