This is being repeated over and over again on this page, but it's factually untrue… I've tasted dozens of kinds of wine grapes, and lots are tasty or at least as palatable as some of the blander table grapes – some are even widely recognized as such (muscat!). Also, even varieties that are not great for eating can often be used to produce some very nice juice – cabernet sauvignon, merlot, chardonnay are all used for high-end, monovarietal grape juices in France for instance.