In SC1, Protoss players completely ignored shield "armor" upgrades. And with good reason: they were worthless. Shields take full damage from all attacks, so armoring them was meaningless. What good is turning a 20-point Vulture attack into a 19-point one? And even though shields heal (relatively) quickly, they run out just as quickly, especially to Terrans who use a lot of damage types.
In SC2, shields work differently. They take damage as the underlying unit does. They heal extremely quickly if the unit is out of combat for long enough. High Templars now have a spell to help heal shields (though the particulars of the spell are unknown). Shield upgrades are not broken down into "ground" and "air"; shield upgrades cover everything, even buildings.
Immortals take this to a new level. They get to apply the shield "armor" after minimizing the damage to 10, so each +1 of shield armor reduces the maximum damage they take to shields by 1. At +1, you need 11 separate attacks maximum to punch through their 100 points of shields. At +3, this becomes 14. This is on top of having 200 points of Hp.
My question is this: does all of this effectively compete with +1 armor upgrades, or is it actually more powerful than the armor upgrades?
My concern is particularly for TvP, where Immortals with +3 shields can be devastating. It even makes Marine attack upgrades past +1 meaningless. This becomes even stronger if you've got a HT or two doing their shield healing thing on them (though it might be best for them to take the opportunity to Psi Storm the Marines instead).
I know units don't have that much in the way of shields compared to Hp. But still; fast out-of-combat shield healing + standard damage + serves air and ground units equally could make armor upgrades the meaningless ones. Which effectively means that the Protoss only have 3 domains to upgrade rather than five.