I have fitted a second set of indicators and brake/tail lamps onto the back of the roof of my Landy.
I don't think the existing small round units are very noticable, especially if the vehicle is dirty. By adding on these extra lights it makes the back of the vehicle more easily visible in the dark.
Land-Rovers are regarded as slow vehicles, so I do have a problem with people tailgating me. Even if I am absolutely screaming along at a totally inappropriate rate of knots on any particular road, cars drivers approaching from behind sometimes feel that they absolutely have to catch me up and sit half a cars length of my back end to prove that they could be going faster than I am if only I wasn't there!
When someone is tailgating me, it is nice to know that the high level lights will shine over their roof, so at least any car behind them will still be able to see when I am indicating or braking.
Actually, I haven't got around to wiring the lights in so they work yet. I need to buy and fit a heavy duty flasher relay, because when I am towing my caravan I'll be asking the flasher relay to operate 4 light bulbs on each side of the vehicle and trailer combination.