I see no people using it nowadays, all my friends that play with EK just have it for decoration.
Shields like Gnome Shield and Falcon Escutheon have bigger Def and more % of elements comparing to the Ice form of Rainbow Shield (it's form with the best Def of Rainbow Shield). Another reason is the rainbow shield after being enchanted, it has duration only 15 minutes, so it's not functional.
In my opinion, only it's Sparking Rainbow Shield could be useful to give more speed to run to somewhere.
Icy Rainbow Shield
(Def: 39).
It can only be wielded properly by knights of level 100 or higher.
It weighs 69.00 oz.
It has been temporarily imbued with ice magic and has a greatly increased shielding value.
Sparking Rainbow Shield
(Def: 36, speed +10).
It can only be wielded properly by knights of level 100 or higher.
It weighs 69.00 oz.
It has been temporarily imbued with energy magic and increases your speed.
Gnome Shield
(Def: 38, shielding +2, protection physical + 6%, energy +8%, ice -2%).
It can only be wielded properly by paladins and knights of level 200 or higher.
It weighs 60.00 oz. Imbuement Slots: 1
Falcon Escutcheon
(Def: 40, protection physical + 7%, fire +15%).
It can only be wielded properly by knights and paladins of level 300 or higher.
It weighs 60.00 oz. Imbuement Slots: 1