First of all the virus sets up a factory to generate copies of itself. The cell is then ruptured releasing the viruses to infect other cells. So the cells would be destroyed regardless. The free viruses can be destroyed before they have a chance to infect other cells. And would destruction of some CD4 cells be all that detrimental? Not really. People can have zero CD4 counts and still be healthy otherwise. This is because the CD4s are not the only immune cells and they do not govern the rest of the immune cells. This is why HIV cannot cause any diseases in man. It cannot collapse the immune system.