The main reasons for using the eave and ridge vents are two-fold. First, allowing the air to be admitted at the eaves and exhausted at the ridge utilizes air convection to its full advantage and since the entry point of the air is down low in the attic and the exhaust is high, it will do a great job of keeping the air changed in the attic, thus lowering the attic temperature as much as possible in the summer months for a "passive" ventilation scheme.
Second, admitting air at the eaves keeps the back side of the roof sheathing cold in the winter. This is a typical design feature of "cold roof" construction. The theory is that with the roof deck being kept cold, the layer of water that commonly forms under ice on the roof is more likely to stay frozen and not wick its way into the building. Now of course the roof will stay cold as long as there is no source of heat in the structure. Since we are dealing with a heated structure, the trick is to keep the roof deck cold. The more insulation you have to keep the heat from the roof deck, the better.
It would be helpful to know what your insulated envelope will look like. In a gambrel for instance, you would commonly insulate the vertical "knee wall" and then either a flat ceiling or a cathedral ceiling. I am assuming that your eave vents are at the lower portion of the gambrel roof and not at the start of the gable roof. In that case, with an insulated knee wall the entire area behind the knee wall will stay cold and the only place you will need the baffles would be at the junction of the two roof slopes on up to make sure air from the eaves gets to the ridge. That would be in the case of a cathedral ceiling design, If you opt for a flat ceiling, then you only need to ensure the air has passage from the beginning of the gable roof to a point above the top of your ceiling insulation.
The flat ceiling option may be the least expensive in the long run because, while you may not get lots of insulation at the point where the two roof pitches come together, you can insulate the knee wall with any thickness of batt you want, even a 12" bat since it sticks out in the area behind the knee wall. The same holds true for the flat ceiling, you can load that too. This construction will maintain a cold roof design and insulate the structure well. Since you can do the fiberglass install yourself, you can have a well insulated structure for less money than a spray in foam job. If you have any more question, just post.
