There's a very interesting read here by a guy who does medieval jousting reenactments, whose practical experience indicates that a lot of common assumptions about stirrups are somewhat incorrect. As the author notes, few archeologists or historians have any real practical equestrian skills, in particular those involved with mounted combat.

The author and others have had no problems jousting without stirrups, eg. being able to knock down a 200 pound quintain without being unhorsed. It seems that the saddle and horseman's weight is far more important than stirrups and the weight of the horse. The stirrup aids with control and would be useful in close combat where it would make dragging a rider to the ground more difficult, but has little direct effect on the actual moment of impact during a lance charge.

The theory he suggests about the stirrup is that it allowed the rider to more easily rise from his seat, something useful to help alleviate the strain of riding over long distances, but more importantly for mounted combat it also helped to provide a stable 'platform' while the horse is moving, something that would be of great use to a horse archer. As he comments, it's noteworthy that the stirrup appears to have originated among cultures known for their mounted archery.

His comment about a 'page' being accidentally struck by a lance at a jousting display, and getting thrown back some 15 feet made me smile considering the lack of impact of the charges in MTW.