Maybe you're looking at a different book than me, but the stats on the magic shields in my book don't scale with their item level.
This statement confuses me, as all the shields in the book which have more than one level of item they exist as have improved stats at higher levels.
Are you referring to the examples of magical shields which have other magical traits? If so, you're talking about something that is roughly equivalent to the statement "the stats on antimagic armor in my book don't scale with their item level." Because in both cases the stats you are talking about are scaled via another source (potency runes for armor, and special materials for shields).
So unless you're suggesting the DM send level 1 enemies against the players for the whole campaign...
Back away from the extremes, they don't help anybody.
I was talking about the difference in damage output potential when comparing one campaign that trends toward lower-damage enemies (such as via trending toward low-difficulty encounters) where even a "fragile" shield can find opportunities to block attacks without being destroyed, and another campaign that trends towards higher-damage enemies (such as via trending toward high-difficulty encounters) where even the sturdiest of shield is likely to be pushed to its limits.