I can see advantages to it, and disadvantages. I've drilled and tapped my bandsaw table top for jigs, so I would prefer a bandsaw table of cast iron. I haven't had a reason to do the same to my tablesaw, and a stock feeder is the only reason I can think of. I don't really have a rust problem, so I'm ambivalent about it for a tablesaw top. If the rest of the saw is worth the money then what the heck. I'd rather see riving knives, a nice easily removable blade guard, and good dust collection. A granite top is waaay down on my list of features I'd pay extra for.
Like Barry, the jointer fence would be the thing I'd like to have in granite. Those things don't seem to want to stay straight.
Jay