If the installation fails before reaching completion, the user could be left with an unbootable system. It would be nice if anaconda could install base packages (small internet applications, base utilities, yum) and the bootloader before installing the rest of the packages requested. I had this problem when bootstrapping an FC install from Windows. A network error meant that the installation failed to complete, and I was left with an unbootable, half-installed system.
The problem comes in with where do you split things. Plus, stopping in the middle of package installation to install the bootloader makes things a bit ugly also.
There are too many issues trying to implement this. If the install fails during package installation, it could be in any number of states. What if rpm failed? Could it be a hardware failure? Network connectivity died? And so on. For all of those situations and more, we offer the rescue CD. And now, we also have the Live CD which is even more fun-filled than the rescue CD.