During maximum braking, your rear wheel is unweighted and unable to provide any braking power. This is, of course, for a nice dry road going in a straight line. As others have pointed out, if the road is wet, especially during autumn, you risk having your front wheel slide out if you try and do this. On wet or icy roads, you need both brakes for max stopping power.

This is also true when you are going fast on a hard turn, which is why road bikes have both a front and rear brake. On a hard turn, you need to keep weight on both wheels so keep from sliding out.