On a hip roof home: ridge vents in conjunction with soffet vents make a super combination for good ventilation.
On a Eve roof home: they alone do a good job of circulating air in and out of an attic. Even with the aid of wind driven turbins, they work grreat. The problem with most eve vents are that they get all conjested with bird's nest and debris and there for cannot breath properly and the same with soffet vents, most of them are covered over with insulation and do not draw.
Eave vents with ridge vents even they may do exactly what you stated will still exchange air and cause a movement up there which is what you want. But, I know that ridge vents are expensive and if you always consider every thing that you add to your home in the ways of energy effecience, consider the time it will take to pay for itself in savings. The quicker the better.