There is, mostly in the Road forums, a rabid group of misinformed people who will tell you (almost) that bad things happen when you use the rear brake on your bike. Oddly enough they are as afraid of using the rear brake as some...again misinformed...people are of using the front brake. People who are afraid of using the front brake at least have a reason, however invalid, for their fear of using the front brake. The rabid anti-rear brakers only reason is because it's uncool and they misinterpret the physics.

As I've discussed in Andy_K's thread, the logic...even Sheldon Brown's...is flawed. The maximum deceleration you can obtain is at the point where the bike is about to rotate around the front hub. It is not when the rear wheel skids and you are still getting some braking from the rear tire or even when the rear wheel is lifted. You have to be up in the air just about to go over and onto your nose. This is the maximum you deceleration you can obtain because there isn't anything more you can squeeze out of the system past this point. You are no longer braking but falling.

What people who adhere to the 'you get the maximum out of your brakes by using only the front one' don't realized is at what point you get the maximum. If you are going to hoist the rear of the bike into the air and balance at the tipover point each and every time you use your brakes, you'll get maximum effectiveness out of your brakes. You'll also have a huge dental bill. It's just not practical to try to reach the maximum each time you want to stop. It's probably not practical to try to reach that maximum even in a panic stop situation. Up to that tipover point or, at least, up to the point where the rear wheel loses contact with the ground, you improve your braking by using both brakes.

In fact, you can vastly improve your braking ability by changing your center of gravity on the bike. The further back and down you push, the harder it is to lift the rear of the bike and the more effective your brakes are. The first mountain bike rider to put front brakes on a mountain bike learned this technique and it's almost universally practiced by anyone with some mountain bike experience. You'd never hear this 'use only the front brake' argument from someone who rides off-road. If a mountain bike rider doesn't have first hand experience with a stopped from wheel and an endo, they are either new to the sport or they ride only in Kansas*

*Yea, I know Kansas isn't flat but most people think it is. Sorry