Having a headlight out is dangerous. Not only can you not see properly in an unlit road, but a defective light means that other road users will have difficulty spotting your vehicle properly or knowing what it is. In the rear mirror, the ‘one-eyed monster’ might be mistaken for a motorbike. A single headlight also makes it difficult to gauge the width of the oncoming car and increases the possibility of a ‘scrape’ or could result in a misjudged overtake, especially if the offside (right) headlight is not working.
A defective rear light raises different problems. A brake light not working will make a rear end shunt more likely as the driving following takes longer to realise you are stopping.
It’s also an offence to drive with defective lighting yet checking your bulbs only takes a moment. In slow moving traffic you can see if you have both headlights working by studying your reflection off the car in front. And if when you park you are by a window use your mirrors to check that the brake light and red tail lights are working. If in doubt have somebody watch as you test them.