well, inittab is how it should be.
Because if you use the rc.conf daemon method and your display manager always hangs because of some trouble, you'll have to boot into run-level 1 via grub, remove gdm daemon, and then reboot to fix things up. You're firing something up when it shouldn't get fired. I prefer booting into run-level 3 via bootloader to fix problems; kdm/gdm as a 'daemon' hanging in the beginning won't let me to.
Or (All iz well scenario) in simple words if you fire them as daemons, you can't prevent graphical login i.e. can't go to run-level 3 via bootloader.
Like I said, inittab is how it should be. This is a weird answer. But still inittab is how it should be. imo, starting your display manager as "daemon" is a "hack"... I hope you got what I meant.