What ends up happening is that the user is presented with something that looks like it's ready to be used, but isn't. You may be missing your networking or not have file sharing, plus your system is using up all it's I/O and cpu, which means that it's not going to be responsive or open up your applications quickly.
A extreme example of this sort of approach would be just posting a screenshot of the desktop as the splashscreen and let the parts fill in and replace bits of the screen as they are loaded. It could be a neat effect, but it would be a lousy way to hide the fact that it takes many minutes to boot up your system.
Plus the point is to have the system fully bootable. So it's a challenge, a sport, so you have to have rules so nobody tries to cheat. :) Now if you have a system that boots under five seconds, but shaves off a second or two by bringing up a fake-ish user interface then that may be something else.