I've been on clipless pedals exclusively on my road bikes and mountain bikes since 1987 (including my trials bikes and FGs), and I feel uncomfortable without them. But you will hear from a lot of riders who never use clipless, and often it is the downhill specialists and ex-BMXers who lead that pack. There is no question that clipless pedals really benefit you when climbing or riding out of the saddle though, and it seems like more singletrackers prefer them as well, but what it really comes down to is personal preference.