There's a number of posts going about on the blogosphere about the piddly little 1 point bonus AC award in most D&D and D&D-like games give to the shield. I've done it myself in the past in these posts : shields again, splintering by material and shield combat variants .
All those options were pretty much geared to action in man vs man combat where armor class modifiers vs armor type were unaccounted for (which were present in some earlier forms of the game).
One consideration in the estimated value of the shield that is often ignored is combat outside of the theater of man-to-man combat. A heck of a lot of RPG combat is man-vs-monster and the monster is often little if anything like a man. The strength, size and fighting methods of many a monster would limit the effectiveness of a shield greatly.
Inhuman Strength and Size would easily defeat the shield as defense. A nice big target for a war-mammoth to crush isn't much of defense is it? An opponent who strikes you hard enough to splinter the arm bearing the shield is little inconvenienced by the shield. A big giant would surely be able to punt a shield warded warrior rather easily.
Shields aren't really much of a barrier for attacks they weren't designed to block. Explosive spells detonating on the surface of your shield might be a little better then having hit you at all but it'd sure be nice if they missed you entirely. The touch of a wraith or spectre is little interfered with by a bit of wood and metal interposed by a doomed victim. Is there really an advantage to holding a shield when green slime drops on you?
Tiny opponents don't' much care about your shield. A swarm will climb over your shield or dodge under it with little difficulty. diminutive warriors can take advantage of your limited reach and visibility inflicted by your own shield.
Maybe when we really look at the wide range of attacks a D&D combatant actually has to deal with a 1 point bonus isn't all that unreasonable even if many of us can agree the shield is much more effective in actual man-to-man combat.