protection and shields

[Taqwus]

I wonder if shields actually should be useful against, say, unusually powerful projectiles. Hurled boulders, Gifts from Heaven (do shields affect counter projectiles? Hm.), even castle ballistae shots perhaps should not be readily blocked by a shield. Or perhaps give a deflection possibility, but also a chance of crippling based on target's strength or hp.
Another bit is that perhaps large shields should confer some benefit to others in the same square. In the ancient world, weren't there soldiers who carried extra-large shields to protect archers or crossbowmen? Perhaps among the current shields only tower shields should confer any significant co-occupant bonus whatsoever, but *shrug* it might make sense.