Do I Need a New Hub?
I tuned my Ultegra 6500 rear hub this spring. While cleaning it I discovered a defect on the bearing surface of the hub. I couldn't see anything, but it felt like a miniscule deposit. I couldn't remove it with my greasy fingernail. I regreased the hub and put it back together. As I was tuning the hub I could feel the defect when I spun the wheel with the cone nuts a little too tight. When I got the right tightness, the wheel spins smoothly with no slop. Just for information, the hub has about 600-800 miles on it. I had the axel readjusted a couple of years ago by my LBS to the narrower width for an older frame. When I initially took the rear wheel off (this spring), my impression was that the cone nuts were quite tight, too tight even.
If I need to replace the hub, can I reuse the spokes?
If it was my bike I'd probably take some polishing compound and see if I could buff out the defect.
If you decide to replace the hub, the spokes will most likely be the right length because most Shimano hubs have similar dimensions. If I were doing it, however, I'd still get new spokes. Relacing a wheel is a fair amount of work. Disassembling the old wheel and cleaning up the old spokes adds to the amount of work required and you'll probably have to bend some of the spokes in the process. It's more work, your result will be marginally less reliable, and the cost savings compared to the time and effort required isn't that much.