+1 to rabbit, liza and grog.

On descents, you want your weight to the back of the bike anyhow, and braking safely is only part of the reason (handling is most of it).

I'm honestly not sure if there is such a thing as going over the handlebars. On motorcycles, it just plain doesn't happen, but it's a huge myth that many people believe. What there is on motorcycles is what's known as a "highside," where the rear wheel loses traction and then regains it, which causes the rear of the bike to buck into the air. Away from the racetrack, it usually happens when someone uses too MUCH rear brake, locks up the wheel, then panics and lets off the brake.

One exception to the rule of emphasizing the front brake is where traction is iffy (such as on gravel and very wet roads). There, you do want to emphasize the rear brake, since you're basically planning to skid, and a rear-wheel skid is much easier to control.