I would go with out my back break before I would go with out my front. I agree with the above statements "the front does most of the work". When you apply the front break the weight transfers forward and then down (front shocks compress), driving all of that momentum/force back down into the pavement (helps your traction/keeps rear from skidding). As long as the road surface is dry and clean, I feel like I can stop quickly w/ both breaks. With just the rear my confidence level drops considerably.
Is it a show stopper? To each is his own. I would most likely pass...and I am not saying I don't ever take chances, but the older I get I don't feel so bullet proof anymore. Definitely be extra careful if you do. As stated in other posts "Live to ride another day!"
Ru