As mentioned. Bearings and cones are cheap to replace. Shimano insists that the cup and cone bearing model is the best and suffice to say, they can be falling apart and still function for the most part.
I've personally never seen a cup get too badly worn, but cones? They pit really bad sometimes.
The worst thing about the rear hubs is that the freehubs get sloppy and they are almost as much as a new hub for replacement parts and finding the part can be hit or miss.
A good set of hub tools is really helpful too.