It's true about the majority of your braking happening on the front, but that's straight line braking. Using only a front brake while diving into a curve/berm will produce understeer, meaning that the wheel might lock up and slide and not guide you through the curve.

Edit: Also, using front brake only on a trail descent is a recipe for a forward roll. One rock or root and your are rolling.

On trails, your rear brake is used to help control the bike's speed more so than stop quickly.

I would suggest 2 brakes or at least a rear brake before suggesting using only a front.