You get plenty of curved rails when you buy sets for beginners (4512,4511,4564, etc.). When you have got some sets for beginners, you don't really need them as much as straight rails. But you need quite many of them in combination with cross tracks. So when you buy cross tracks and you don't have many sets for beginners, you might need them.

It would be good if LEGO had produced curved rails with different radii. Then you could build more complicated track layouts. That's why I didn't give it 5 points. The quality of these curved rails is as high as the quality of 4515, of course.