This question has been asked before, and is a contentious issue. People are passionate about this on both sides.
I have no personal gain here one way or another, but here is an interesting article on the subject. Note that most homes here in The Villages have met the manual J specs on ventilation with the ridge, below ridge and soffit vents. Many factors must be considered here, such as amount of existing ventilation, color of roof, etc.
Don't Let Your Attic Suck | The ASHI Reporter | Inspection News & Views from the American Society of Home Inspectors
Hope this helps,
Frank