The log-in screen should be shown as early as possible (before even X) and should not block the boot process. Thus the computer would be usable earlier, because the boot process would continue while the user is typing her password. This is especially important for users who are slow to authenticate -- some users take about 4 seconds. With a normal boot, they would wait n seconds to see the log-in screen, spend m extra seconds logging in, then wait d seconds till they can use the desktop. If they can authenticate after x second while the computer is booting, they only wait max(n - x, m) + d seconds to use the computer.
With a long boot, this also allows users to authenticate themselves whenever is convenient for them, rather than waiting to do it at the right time for the machine.
And, knowing which user is logging in will allow further parallelization of loading the user's desktop components and services. That will reduce the time "d" required to start up the desktop.