I don't think you are talking truss rods here, which are adjustable and common in guitars, but rather reinforcement rods that are glued into the neck and are static. The modern composites that are commonly used as neck stiffeners are frequently lighter than the wood itself yet many, many times stronger.
You'll find reinforced necks in the best of customs for the reasons mentioned above dealing with changes in temperature and humidity. Wood has a nasty habit of changing over the years and a reinforcement bar will keep it pretty stable. Most of my necks are Spanish cedar, an extremely light wood. The carbon fiber composite rod I epoxy into my necks make them very light, strong and stable, the three qualities you want in a good neck.