Just so we're clear, there is no "dead cured" rock. Dead rock is dry. Live rock is dry rock that has been "cooked" or "cured" so that it is populated with bacteria that have removed dead organic matter from the rock (during the curing process).
If you put dry (dead) rock into your tank and start it up, your tank will go through the Nitrogen Cycle - which is essentially what happens when you "cook" your rock, but you are instead doing this in the tank.
The reason it is a good idea to "cook" dry rock is because of the amazing amount of gunk that will be released from it. Most processes involve putting the rock in a bin with saltwater, a power head (pump), and a heater, and letting it circulate for a week. Then you take each rock out and dunk it in a series of 5 gallon buckets to release the waste matter, and place it in another bin of new saltwater and repeat the process. You will be stunned by the amount of junk that comes off the rock each week. Some people do this for several months. The result is Cured Live Rock that is relatively free of organic waste matter, and populated with bacteria, ready to put in your tank.