Flat pedals are great for technical riding and if you have to get a foot out to turn really quick. Also if you do a drop or hit a log, its a lot easier to ditch your bike and prevent yourself from getting racked by the seat or handle bars.

However, remember with flat pedals your only getting half of the efficiency since only down-strokes count. And its a lot easier to lift your bike up when hoping instead of trying to bounce up and explode forward. Most you can pop out of fairly easily, but when it comes down to split seconds it can be tough.

Personally I disagree with the idea they are safer. They might prevent your ankle from getting scratched, but if you go off a drop and your going to crash or you botch the landing, you either ditch your bike and roll with a few scrapes and bruises, or stay with it and end up in a stretcher. But if your not doing jumps and drops, and your not dodging trees clip-less are great.