In 2 wheel drive the front driveshafts, diff and prop are all driven by the front wheels, adding friction and momentum changes which makes the vehicle less efficient (ie thirsty). Free wheel hubs allow the front drivechain to remain stationary, improving your MPG and lightening the steering a bit.
If your swivels are oil filled (as opposed to grease) you'll have to run the vehicle with the hubs engaged periodically to lubricate the top bush. You'll also have to engage the hubs to get 4wd - not fun if you're already stuck in mud. I had some on mine, they leaked all the time and, 'cos they stick out, I snapped one off on a rock, losing front wheel drive completely. (just before snapping a rear half shaft in the middle of Bath. But that's another story.) I replaced mine with plain hubs.
I suspect you'd need to be doing a fair few miles of (relatively) high speed travel (for a landy) to see any real benefit.