I think personally, that the two latest responses in this thread are probably the most apt in describing the balance and importance of discipline. On the one hand, a man by rule can not withstand the charge of a heavy horse, without taking very drastic measures (Sturdy spear fixed on a prone position, caltrops, ditches). An infantry line will usually depend their success on the depth of their lines. Shields would have little to marginal effects in disrupting formations of heavy horse, and could on the other hand cause other injuries as a liability for infantry. It would more or less rely on how the shields were locked together. It would simply depend on the discipline and tactics of the horsemen. If the heavy horse moves in a serried, tight formation, by rule the shields of opposing infantry would not be very significant to the defence, lest it was used in combination with other tactical measures, such as the aforementioned ditches, caltrops or fixed spears (Or a combination of them all). If we speak of light horse or a squadron of cavalry too impetuous to maintain formation, then locked shields could carry some merit in deterring individual horses from crashing into a large object (This is the key argument in using a shield-wall; In normal cases horses do not crash into walls or into dead halts, or even into objects larger than themselves, irrespective of training; The shield-wall therefore had a psychological property to it which had effect against cavalry).

On the other hand, even if the horse was at times an unpredictable animal, a full-sized horse as used by Eastern heavy cavalry, standing between fourteen and fifteen hands tall, some extraordinary animals probably as tall as Akhal-Teke at sixteen hands, extremely muscular (Think a compact Percheron at fifteen hands; Cappadocians and Nisaeans. Armenian horses are even more compact but still retains these properties). An average Percheron weighs about 900 kilograms (About 1900 lbs.). So it is reasonable to think that the behemoths bred by the Partho-Sassanians could easily weigh more than a thousand pounds. This is an impressive creature; Training it to carry an extremely heavily armoured rider and heavy barding, and you have got a much heavier horse, yes more encumbered, but nonetheless a unit meant to crash into front lines. To emphasize their purpose, they are trained to amble or trot in a serried and columned formation, maintaining themselves as a mobile fist. Given how Graeco-Roman sources have embellished Partho-Sassanian heavy horse as fearsome warriors, there must be some truth to their reputation.

Then we get back into the issue of mentality and the contest of balance as LorDBulA mentioned. It may not apply as much to the cataphract due to armament and tactics (Instead the cataphract has its own inherent weaknesses, amongst these heat-loss and stamina). The cataphract was conceived in the beginning to counter horse-archery, but developed into becoming the decisive hammer which did not immediately require the anvil (Pin-and-flank). The cataphract, in the form we all recognize given the ludicrous nomenclature (Super-heavy cavalry), was meant to decide the battle with a single charge to the front. Given that this purpose prevailed for centuries, there is no reason to rule out success.