Well, the part you quoted just says to cap the ridge vent, not how much vent should be installed. I agree that they all should be vented, for aesthetics if nothing else.
What other type of venting do you have? Continuous soffit? Gable?
You can't really have too much ridge vent (outflow), but if you don't have enough intake you really don't gain that much I don't think.
Whats the square footage of the attic space? How many feet of ridge vent do you have? What other venting do you have?
Take a look at post #2 in this thread. The links have some very good info.
Attic Ventilation