Shield seems to be made on the assumption that the users will have low AC, just to compensate, but since there's no real limit to casting on armor besides proficiency, a Wizard can go Cleric 1 and have 20 AC base with Plate and a Shield, 25 with the Shield spell. Or without multiclassing, take a race with armor prof and have 19 base, 24 with Shield.It's more limited for Eldritch Knights and Hexblades since they get fewer slots, but not very well thought out for the normal full casters.
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