Most cars don't come with indicators that a light is blown. It's unsafe to drive around with non-working lights, mostly because it interferes with other driver

Most cars don't come with indicators that a light is blown. It's unsafe to drive around with non-working lights, mostly because it interferes with other drivers' expectations (working brake-lights and working tail-lights, light geometry allows guesstimation of car dimensions), but also because it can impair the driver's own visibility. But presumably these could be dealt with friendly warnings, and not a cause for fines or arrest. However, once the traffic stop is made, the officer has wide berth on how to respond, making it ultimately very subjective.