Shield AC for a caster

[HanzzYolo]

I'm curious how effective a shield like "A Shimmering Orb" (15 AC, +20hp) is at reducing max hits. Ive heard shield AC is far more effective than other AC at reducing max hits. In my mind, that could be very helpful dmg reduction.

For example, you are getting quadded for 180 a hit. If one of those 4 hits only lands for 90 dmg, you are saving yourself 90 hps. Two rounds you save 180 hp. ( All hypothetical, and best best case scenario).

Now imagine a secondary item (lets say a stein) with no AC, buy a +90 hp bonus. This thing would probably sell for 5k. A shimmering orb sells for 100pp.

If you are following, it seems like caster shield AC is undervalued. I assume im missing something, or valuing AC too much. Most casters i see do not bother with shield.