I don't think there's any question that a firm attachment to the pedals is necessary for racing. But it's not about "efficiency", it's about getting power into the bike, and getting maximum power into the bike means using more than just the downstroke.

If we, as tourists, spend a lot of time putting out maximum power, then clipless (or a proper clip/strap/shoe set-up) will be very helpful. that's not how I personally tour, though, so I find a firm attachment to the pedals to be pretty unimportant while touring.