I usually wear running shoes, and in these you have to yank the strap uncomfortably tight before getting out of the pedal is a problem. I usually don't tighten my straps anyway, in which state my shoe just slips out and gets in with a couple of wiggles. For rides longer than ten minutes or so I'll tighten the straps fairly firm, and getting out of them is still easy once you know what it takes - just point your hell down and twist your foot a bit while standing on it. I never have to loosen my (cheap nylon) straps by hand, either - I can just loosen them by twisting my feet.

I do all my riding around town in fairly heavy traffic, and there's no way I'd feel half as safe without toe clips. Being secured to the bike is a big plus when you can disengage with only a minor effort. The biggest downside of toe clips is the entry technique you have to master... it's a fiddly one.

I don't think 15% is a conservative figure for the difference - without foot retention you have to tell your muscles to keep your feet on the pedal, as well as worry about smooth power delivery. The latter will suffer when you have to do the former, and we know that power delivery that isn't smooth is inefficient power delivery.

There was a camp who felt that given the hours road riders spent on the bike, that the concentrated load on the sole from the small SPD cleat could cause potential soreness and fatigue compared with the large LOOK style's far greater area... the stiffness of the soles in road cycling shoes somehow notwithstanding.