"Energy Efficient Housing -- A Prairie Approach" is the only book I know of which advises against this practice (check out the graphic, keyword "ventilation", title "WHAT IS A CONTROLLED COLD-AIR INLET?"), so I asked the Saskatoon Research Council why. They are not against the practice in general. In the context of super-insulated and super-sealed houses, the subject of their book, such ducting is not harmful but wasteful. Thus, if your house is super in every other respect, the cold-air inlet should not go directly into the furnace ducting but first pass through a Heat Recovery Ventilator to save even more energy.