I feel motived to tell my story on the subject. I got my first (and only) road bike in 1985. It had toe clip pedals. I rode it in what ever sneaks I had on my feet, mostly running shoes. Friends showed me their clipless, stiff bike shoes. They couldn't walk in them for sh**, but they really did help when pedaling. I found really stiff sneaks and said close enough. When I bought a mtn bike in 2013, I decided to go with flats, since I have never owned clipless. I firmly believe they are better and more effiecient for bike riding.
My flats, though, were the right choice for me. The ability to get off the pedals instanly has saved my ass more times than I can count. I have only fallen off my bike twice, both at almost stopped speed. NO damage. I can't count how many times I have landed on my feet while my bike flipped underneath me. If I was on clipless, I would be flipping with the bike. I ride really rocky terrain on a hard tail, and bail off my pedals at low speeds often. I don't do crazy sh*** , but I am not afraid to give something a try. I go slow and if I get out of control, I put my feet down and hang on.
I definitely loose pedal efficiency and if I was doing a long uphill climb, I would rethink it. If you ride single track with a lot of up quick up and downs. flats are safer, no question. If I am climbing a mtn, I would be on clipless.
For shoes, I have some 5/10's, they are almost like a ski boot, and really protect my feet from rocks. I can walk in them comfortably, especially on rough terrain. I have no second thoughts about them or desire to get something else. They are super sticky and I never come off my pedals. (Canfield crampons) I have watched plenty of my friends who ride clipless fall off their bikes when they don't get out of them fast enough. I am new to the sport, and like the safety over the efficiency. It really is personal, and its pretty easy to figure out which camp you are in. Having said all this, I have never tried clipless and now that I have developed some skills and know my limits, I wouldn't mind trying clipless. Maybe if I rent a bike sometime, I will rent clipless shoes as well.
It would be interesting to hear from someone who rides both, and I think we have in this thread.