The big reason most games frown on crossbows/shields is because firing a crossbow takes two hands, and most games don't account for the specialized shields archers use in formation fighting. In part probably due to lack of research, in part probably due to a dearth of formation fighting.
Hopefully once that makes the game (sometime in the Army Arc, I believe) it will be in a tactic in DF.
Some of the best-documented crossbow units were the
Genoese crossbowmen: "Apart from the crossbow, they were equipped with a dagger, a light metal helmet, a gorget, a hauberk and a large shield, called a pavese (pavise), which was used while reloading the crossbow." The
Pavise was a fairly large, reinforced shield with a flat bottom, and frequently with multiple carry handles and/or a spike on the bottom.
The
Battle of Crécy was one of the most important battles in the Hundred Years War(s), and notable in that the English had longbowmen, and the French had hired mercenary Genoese Crossbowmen. The French had marched everyone hard, and the crossbowmen's shields were ordered to be left behind in the baggage train. With a rate of fire of 1-2 shots per minute normally, but tired and without any natural cover or their shields, the crossbowmen were seriously outmatched by the longbowmen, comparatively rested and capable of 5-6 shots per minute at longer range. Additionally, the crossbowmen had been marching at battle ready during a rainstorm, whereas the longbowmen had cased their bows until the last moment, risking that the high ground and improvised earthworks would give them time to string and fire.
In general, Crécy is an excellent example of how a nominally superior force (French: 35,000 to 100,000 men, including a large force of elite knights in the best equipment of the time; Anglo/Welsh: 9,000 to 10,000 men, far less armored on average) can make a series of bad strategic and tactical decisions and end up humiliated. In games that don't simulate the field level battle in detail or at all, this sort of thing is why random result tables have some seemingly unlikely results; in games like DF with the potential to simulate conditions in detail, hopefully the Army Arc will allow us to win, or loose, based on our ability as commanders as well as by being backed by our arsenal of dwarfocracy.