Building on comments on my post yesterday -- In my experience with western martial arts -- which is, admittedly, limited to longsword fighting from the 14th-ish century -- a swordsman doesn't use a shield. Instead, he uses his sword as what (3.5e) D&D would call a "hand-and-a-half" sword: he switches from one hand to the other as necessary, sometimes uses two hands on the grip, and sometimes uses his second hand to "half-sword," putting his hand on the blade (it's not particularly sharp in the middle, and won't cut anyway without the power of a swing behind it) to use the tip as a dagger in close combat.
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. Neither of these situations really seems to describe D&D combat particularly well. My vision of it, at least, is a lot closer to the middle ages styles that involve a sword during the period of transition between the heavy blade of the dark ages and the light fencing blade it eventually became in the Renaissance.
Granted, my understanding of medieval fencing is pretty limited, and my understanding of the history of weaponry and western martial arts sketchier still. If someone who has a more thorough understanding of the topic can correct me, feel free to do so. 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.