riding flats or clipped in, doesnt matter to me what other folks use, live and let live is my motto.
however, my lifetime of riding, 30 years of touring, and riding regularly, and using spd's since 92 have shown clearly to me the increased efficiency of clipless pedals--very specifically with climbing.
In tests I did very early on, I found that for climbing, there is a noticeable difference, and I like a lot even on the flats that I can switch up my muscle groups in how I pedal, which is a nice break.
In any large group of people half the group could find something preferable while the other half finds the other thing preferrable. But they've tested this with pro/semi pro racers and in the lab on the flat it makes no difference for them. Here's one easy to find test from youtube in a lab:
I can't say "you" don't, what do I know over the internet?
But I know in actually testing it with people who are very good at biking they didn't find the speed or efficiency advantage people thought they would. I've read other stuff about why, and they think that:
- The big major powerful muscles groups (quads hamstrings) evolved to exert force downwards
- There are small muscles that pull the leg up (hip flexors)- The specifics of how the body works mean you can't actually turn them all on at the same time - you can't push down with quads and pull back with hamstrings with one leg and pull up with the hip flexors on the other. Kinda like how you can't drive your car sideways because it's just not designed to do it.
- There is some debate about whether using the hip flexors in such a manner is really a good idea as overly tight hip flexors cause posture issues but that is a different subject
I figure also the fact that I am not a strong rider, so I tend to notice small advantages.
but hey, again, thats my take on it, and certainly with touring on a heavy bike where one realizes right away that there really is hardly ever any real "flat", I'll take any advantage I can (and throw in all the unloaded riding aspects of clipless that are great--stability flying over rough surfaces, sprinting for lights or whatever--both things I do every day and clipless are great for this.)
The biggest undisputed advantage of clipless is simply foot retention, you can see why people in a race don't want their foot slipping off the pedal:
https://www.youtube.com/watch?v=SVYPpqCvles&feature=youtu.be&t=77
Like I said there's debate on whether clipless might well make you faster (though at the cost of burning more calories) in a sprint. And it might save you a few grams compared to flat pedals.
But the "clipless is inherently faster/more efficient in general and for everyone" is not something they found when they ran tests looking for it.