I had zero intention of ever going clipless when I re-entered the hobby a couple of months ago...but now I won't go back. For many of the reasons have stated.
- GCN video may have shown little-to-no gain, but that's because GCN riders are pros and are disciplined enough to keep their feet in optimum pedaling position. Me? When I go from my road bike (SPDs) to my hybrid bike (dual platforms) and use normal shoes...I routinely find myself failing to stay in optimum form/position.
- By going with the SH56 cleats, I've never had an issue unclipping, even in unforeseen, fast dismounts. My first 10-15 minutes with clipping in was slightly shaky, but after I got the hang of it, I don't even think about it. And, even though I was told the 56s might increase accidental unclipping, that also has never happened to me.
- Related to my first point, I have had my flat shoes slip off of the pedals, something which clipping in eliminates
- The cleated riding shoe is the only hard-bottomed shoe I own that's suitable for riding. All of my other shoes are either work shoes (which I won't ride in) or athletic shoes with soles that flex and give a lot.
As Canklecat says, I haven't done a scientific A/B (and I'm too lazy to switch out the pedals on my road bike) but even if I could achieve the same overall top speed with flats, it certainly feels easier to pedal and sustain it when I'm clipped in. I'd rather be able to expend a little less energy to maintain the same speed, and be able to ride longer. There's a whole host of reasons my fitness bike is harder to pedal than my road bike (pedals only being one of a half-dozen or more factors) but that's why I'll ride my road bike for hours while I get tired of my fitness after maybe 45 minutes.