I can't say I've ever seen a disadvantage in leaving "too big" a gap between me and the car in front.

If someone wants to pass me and then sit up the other guy's arse, fine. I guarantee I'll be ten seconds behind them at the next road junction, or not much more. I don't think people realise that the relationship between road speed and journey time isn't linear - doing 90mph won't get you there appreciably faster than doing 60mph unless you're on a very long perfectly straight road with no junctions to turn off at.