In the winter time, the moisture from the bath area (read: warm and very humid) is dumped into an unheated space. When the Temperature is sub-freezing, the vapor will make it as far as the first cold thing it encounters; could be a rafter, insulation, the roof deck, anything that's cold. When it warms up, the moisture that's collected will melt and dampen the insulation and ceiling drywall. Insulation is way difficult to dry out, and drywall under insulation is virtually impossible.
I suspect some will say that the roof venting will eliminate the moisture, I wouldn't rely on it. Roof vent's are designed to remove excess heat from the attic space.
The contractor may be looking to take the easy way out. If there's only a couple of feet from the roof to the ceiling, I would think he could fasten the INSULATED flexible duct to the fan housing before installing, and when the roof vent hole is cut, he could reach into the attic retrieve the duct and fasten to the roof vent. He may not have to enter the attic space.
Check out these diagrams