My favorite SPD sneakers are NOT stiff, and to me, that's a feature, not a bug. It lets me alter my pressure points. I use SPD-on-one-side pedals, and I occasionally pedal on the regular-old-pedal-on-one-side side, even with my SPD sneakers, to prevent foot fatigue.

In other words, I agree with Petersen 50%. I like being clipped in most of the time, but there are benefits to having no retention. The biggest one for me is being able to vary my position, because I get numbness and occasional pain.