I'm not sure about this, but think about it this way. It takes a lot more gas to accelerate a car than to keep it running at a constant speed. It is much better for a car to keep a constant RPM than to constantly shift RPM's...
Graphics cards could work the same way, but I don't think you'd really see a difference in life of the card since everyone here typically replaces theirs from a 1-3 year period.