I prefer clipless while touring or doing almost anything else bike related. I'm severly considering putting dual side cliplesses on my 'drunk bike' just so I can use my clipless sandals with it.

That said, clipless does not increase efficiency in that you won't be faster with them. They do not miraculously alter thermodynamics so that you'll somehow go faster without using more energy. That's not even the point of clipless. About pro's climbing with every muscle they have... Well, it's true that they do that as well as sprinters use every muscle in their body to sprint etc etc. However every one of those muscles is also eating from the same energy pool and getting their amount of the limited oxygen the body can scrounge up. That's why climbing or sprinting hard makes you out of breath faster. This is really very basic cardio 101 stuff.

Say you use flat pedals and only use two muscles when pedaling. Then you use clipless and use 10 muscles. If you are still putting out the same watts, you're using the same amount of energy and oxygen no matter which muscles are doing the work, no matter how many muscles are doing the work. There may be some very minor efficiency increases, but those may go both ways, so stating that clipless and pulling up is the winner is as likely to be false as stating that flat pedals are the winner. But there's an important difference between the two. Flats don't allow one to use all the necessary muscles in the first place as sprinting for example kinda requires the pull up. Yes, you'll use energy faster with more muscles firing up, but with a quick sprint or climb that hardly matters.

The other pros of clipless that are important to me are
less fatigue (having smooth flats is really fatigueing, having spiked pedals on the other hand tear up your shins)
more safety (when one has experience the foot flies off the clipless pedal faster than one can detatch a foot from a spiked pedal) and to expand on that
- easier to chill when standing on the pedals as the feet are not going anywhere
- feet won't slip from the pedals when the going gets rough
more comfortable
- I can use sandals which still have a stiff sole for pedaling
- don't have to concentrate on my feet at all as they have their set place
more potential power should I need it
- like I explained above
And the absolute most important thing
When stationary I can spin the clipped foot backwards to get my foot into the perfect starting position

Flats do offer some benefits too which is why I use double side pedals on the tourer
I can use whatever shoes I want
- don't have to dig up the cycling sandals if I need to do a quick run somewhere
backup if the clipless system fails
pedal reflectors
can ride bare foot for short distances
the dual sides don't have spikes which is nice