The main benefit to starting it as a daemon is that you can stop it. You might want to do this if, for example, something goes wrong with your X install. You might want to stop it in order to fix something that it better done without X running.
If you kill kdm launched by inittab, init will simply respawn it. If you stop a daemon, it remains stopped until you restart it.
This will only be a benefit for you if this scenario occurs, so I'd stick with your current configuration until you get into a situation that warrants changing it.