It's all about preference. Personly I ride cleats when I'm riding downhill, mybe because I'm used too them because of Cross-Country. Since I got used to them on my Downhill bike I can't imagine riding my bike with Platforms again. I feel like I have more controle on my bike.
My 2 cents.
Obviously its personal preference. I usually use platforms when racing dh because it is what I'm mainly used to. That being said, if its a flatter course where I need to pedal lots, I use clipless. On my jump/slope bike I use platforms. When I race Super D, ride XC, or just trail ridie, I use clipless. Basically, if I'm going to need to pedal, I use clipless, and if I'm racing on steep technical stuff, or just jumping and hitting up the bikepark, I use flats. (Krob, your an ignorant dumbass-not only using clipless on your road bike, but for your general attitude towards them.)