For me, the studies about whether using foot retention increases efficiency or power miss the point. Those factors may be important to racers, but for me using foot retention is about allowing me to more easily spin circles, to more easily use different muscle groups on a ride and to more easily spin at higher cadences without losing a pedal. For example, if I've been spinning circles for a while (so, not pulling up on the pedals, but unweighting, to a greater or lesser extent, the foot that is on the pedal that is on the upstroke) and my hip muscles are getting fatigued, I can switch to pushing more on the downstroke and put more stress on my quads - and, of course, vice versa - without also expending energy/attention on keeping my foot properly centered on the pedal. When I use flats I'm constantly making micro adjustments, and occasionally making fairly significant adjustments, to my foot position on the pedal. I guess I'm a ****z.
Is spinning circles with flats possible? Is spinning a high cadence possible with flats? Yes, of course, but I'm not a pro, I don't cycle 10's of thousands of miles a year, I get tired and sloppy, I'm not the most coordinated person in the world and I'm probably pretty low on the ratio of fast twitch to slow twitch muscles, so having foot retention makes cycling easier and more enjoyable. The occasions where I'm actually pulling up on the pedal are extremely rare (certainly fewer than 1x/month, and only for a few seconds at a time), and I'm not thinking about "efficiency", I'm thinking about raw power to get up and over the last few yards of that brief steep climb, or to beat that light when I'm in a gear that's a bit too high and I don't have time to downshift. Actually pulling up on the pedals probably is a net negative in the long run, but can be advantageous in the moment.
I like clipless pedals, they're far easier to use than clips and straps, and cycling is far more enjoyable for me using foot retention than not. Other people have different experiences, and that's fine. If the OP tries clipless and finds he/she doesn't like them, then at least they know.
(edit: oops, didn't realize that "s p a z z" is verboten)