They are until you need it.
It depends on what it is. On BMWs, it's well worth it. Because they're pricey when they break down. But in order to keep the warranty active, you have to pay for a service check-up every couple years or so, which is like $300 in itself. It's pricey to own a BMW.
But most things, as long as you take care of it, you shouldn't need a warranty, and can just by a new one whenever it breaks.