I did this for the first hour of mod 2 training. My tutor picked me up on it immediately and got me to stop. I couldn't see the point back then - I was slowing down just fine using engine braking and a touch of back brake. But you want to acquire the habit and the adeptness of using both brakes together effectively, because if you do this by habit, then the habit will ensure you come to a quick halt in an emergency. It might help to have a bit of understanding about how the suspension works on a bike... when you brake, the weight of the bike shifts forwards, so that most of the weight of the bike moves over to the front wheel. Initially, this extra weight is absorbed by compression in the forks (which is why the bike dives forward under hard braking) but when the forks have compressed to the point where they've taken the weight, if you're stll braking then then the front tyre gets pushed down into the road, and the friction between the tyre and road increases. The front tyre also deforms a bit, increasing the size of the contact patch with the road, increasing the friction available even further. This is the moment - typically a second or so after you've started braking - when you can use this extra friction by increasing the pressure on the front brake. This slows you down even faster, which moves more weight to the front, allowing you continue braking harder and harder. When you brake using the rear, the same thing happens initially: as you slow down, the weight of the bike moves forward. But this reduces the weight on the back wheel, reducing the friction available for braking. This is ok when you're braking gently on a nice dry road, but should you need to stop a bike quickly, especially on greasy winter roads, then using the rear alone exposes you to greater risks of locking up the back wheel. I think the recommended approach is to use 75% front and 25% back brake (or something like that) for braking on a dry roads.