My understanding of older styles is that shields were mainly used when the sword was a heavier weapon that didn't allow for much finesse, and in large, well-trained units where each individual soldier's use of the shield contributed to the defense of the unit as a whole.
I think Oddysey nails it here, from a gaming perspective:
The main reason I think D&D doesn't favor shields remains that split between offensive and defensive strategies, and honestly it's a pretty good one -- more offense means shorter combats, and shorter combats means more time for the parts of the game that I find actually interesting.
A game with valuable defensive strategies is a game of interminably long combats. Low-level fights against weak armour tend to be pretty snappy; but pit your 2nd level fighters against guys in plate and be prepared for a whole lot of dice rolling: miss-miss-miss-miss... And that's in a fight where the only real defensive choice is which armour you put on before the fight starts. Imagine how ugly it could get if the goblins could form a shield wall or a turtle? I've often considered building a combat system around the rock-paper-scissors of thrust-parry-feint, but again, that sort of focus just makes combats take longer, and that's not at all what I want in my gaming these days.
Art by Albrecht Durer.