If we're going to talk about reinforcing rods, rather than solid construction, then the primary advantage of graphite (or more generally, carbon-fiber) is the strength to weight ratio.
If you used the same size reinforcing rods of steel, the neck would be even stiffer and more stable, but it would be way too heavy. Graphite rods allow you to use a smaller diameter and less weight to add equivalent stiffness and stability, and therefore also allow you to keep more of the wood in place, preserving more of the woody characteristics that many of us seem to prefer, tonally.
Carbon fiber is pretty amazing stuff for some purposes (sailboats happen to come to mind). In fact, I saw something on the web about a year ago, where as part of a research project, someone had built a violin entirely out of carbon fiber - very carefully adjusting the plate thicknesses to approximate the same complex harmonic vibration patterns observed in reference instruments.
Supposedly, this worked pretty well. At the same time, I played a couple each of Zon and Modulus basses (though more than ten years ago now, and only in the stores) and found them to be limited in capability. Yes, you can try to warm up the somewhat cold, sterile, brittle sound (just my opinion) with electronics, but I don't believe you can make it more flexible and expressive.
That said, if you can get the range of sounds you care about, go for it.
-Bob
