You can use more breaking power without flipping over on steep descents if you stretch your arms and back pushing your arse back onto your saddle to counterweight the major stopping force created by your front brake. Rear brake does almost fukk all on steep, fast descents in terms of killing initial momentum. Its mainly for balance purposes . Try braking with your rear only and you ll notice that you will lose speed faster from drifting with your rear tyre rather than from rear braking power without locking up your rear tyre !! If we had ABS systems for bikes we wouldnt have this issue.