I'm reading Grant Petersen's Book, Just Ride at the moment. He has some interesting things to say about clip-in pedals, and I have to say that I'm broadly in agreement with him, that most people simply don't need clip-in pedals. He suggests that racing/sport/performance riding has too much influence over the way that most of ride. As part of his general 'unracing' stance he suggests that clip-in pedals offer little biomechanical advantage, that they lead to greater incidence of RSI, that they reduce a bike's everyday practicality and ease of use in return for marginal performance benefits. They don't allow us to pedal through 360 degrees, they add another hurdle that people must jump to qualify as a 'serious cyclist'. For what? You're only cycling for pleasure or to get to work, not aiming for Olympic gold. If your shoes slip on your pedals perhaps the answer is to change your shoes (or pedals) for grippier ones.
None of my current bikes is 'performance' enough to warrant clip-in pedals, my road bike sold and my mountain bike up on blocks. My posh carbon soled shoes languish under the bed, my Ti pedals somewhere in the garage. And I don't miss them a bit.