User blog comment:Emptylord/Champion reworks/Rammus the Guardian of the Sands/@comment-24420226-20160520000554

Shields do give a better visual feedback to enemies than armor/mr buffs - they better convey when and by how much an enemy has become tankier, but shields scaling with resistances can get out of control. Armor would reduce incoming physical damage and make the effective value of the shield much higher than the tooltip shows. Also, converting resistances directly into a shield allows a champ to severely negate enemy penetration.

Also penalizing enemy players for breaking your shield isn't very fun to play against - being able to eat through a shield should feel rewarding, especially on tanks where they are so hard to break anyways. Nautilus loses his on-hit damage, Skarner loses his speed boost, Sion can't detonate his shield if it breaks too early - here, Rammus creates a lose-lose situation where breaking his shield puts his enemies in additional peril, and not breaking his shield means he doesn't die and continues harassing your carries.