First of all, this topic should really be split into two categories: one for everyday normal street riding and one for track riding. As I have never ridden track I can not accurately speak for why racers may or may not want to use their rear brakes on the track however I can speak about the street riding portion.
Realistically, when talking about straight line stopping as an avoidance skill anyone that only uses their front brakes should be considered foolish at a minimum. Anyone that thinks their front brakes provide 100% of their stopping power, should go back to school and retake some mathematics classes.
Front brakes only account for 100% of your stoppnig ability if you either (a) negate the fact that you have a rear break or (b) it is the only brake you apply at that point in time.
The FACT is, the front brake is only accountable for 80% of the
total stopping power of the motorcycle. The rear the remaining 20%.
This leads to two methods of interpretation (glass is half empty/full kind of thing) at this point. In the emergency situation where you are only using the front brake you are immediately robbing yourself of this stopping power. For many more casual street riders this can certainly mean the difference between life and death. As the rear break is designed as a control mechanism to be used for both balance and direction under these circumstances.
If you want to take the optimistic approach (glass is half full), congratulations! You just added 20% to your chance of having an accident.
Now don't get me wrong and think that I'm against the "rely on your front brakes" campaign. I'm not. I am a firm believer in using them and that every rider should continue to hone their skills.
Since someone else mentioned crash avoidance skills, I just thought I would mention the issue of Engine Braking. Everyone knows how to downshift and how to do if fast and most everyone here can speed down 2 or three or four gears if necessary, but I've seen a lot of casaul, weekend warrior riders that have no concept of using engine braking in combination with their normal front/rear braking as a crash avoidance skill. I've also seen guys go down to many and get get thrown forward in the seat and have the rear start to come around on them. (Using you rear brake will control that.)
even the instructors at the classes I have taken [msf and track] have pounded into my head the front break is your only friend.
I just went through the MSF class this past fall and the only thing they evangelized was using the front and rear in tandem. Maybe the Advanced MSF teaches something different??? I am considering becoming an instructor which is why I know this / was there.