It takes awhile to get used to clipless pedals in technical terrain. When my husband first bought them for me, I accused him of trying to kill me for the insurance. ;) But after a couple of months, it got to the point where I felt unsafe if I wasn't clipped in. It is so easy for a foot to bounce off a pedal, as JJ points out. I got Time-ATAC pedals which are supposed to be good for muddy areas and for people with knee problems. At first, I set them loose and spent a lot of time on our flat back lawn, practising quick exits. It became instinctive.
There are some races I do where it is better not to change shoes for biking. Powergrip pedals are the best compromise we've found. If someone really couldn't get comfortable with clipless pedals, these are not bad.
http://bicycling.about.com/od/equipmentreviews/fr/...