Yeah, just in case you haven't realized how painfully obvious the answer to your question is, imagine this:

You're going down the biggest hill you'll be riding on (for me, this is probably something to do with the Alps). Not only is the grade in excess of 10%, but you're going downhill at over 40 MPH without a guard rail. Suddenly a fleet of soccer moms in SUVs start driving at random, hitting eachother, blocking your path. You must stop to avoid the fiasco, and stop fast, so you pull hard on your only brake, harder than you ever have, and suddenly the cable slips. You pull again, but you just go faster. Three seconds to decide whether you plummet off a cliff or get launched head first into an SUV. Two... One... ... This isn't choose your own adventure, you know... no takebacks.