The difference between a 20lb bike and a 16lb bike is much more than weight. Everything on the lighter and more expensive bike is designed for better performance. Is it necessary? No. Is it nice? Yes. More reliable and responsive equipment certainly helps, perhaps more so with confidence than even in actual race day applications. That's just on the flats though. I lamented riding a rear 404 last week on a mountain climb due to a flat. That was only 5 ounces heavier than the wheel I wanted to ride.