Less codified systems can also create a barrier to entry for DMs. This barrier involves the DM flailing wildly when the PCs do something odd in the first couple of sessions, then crashing and burning and not going back to DMing. Good rules assist DMs, pointing them in the right direction, rather than hiding them. Something both 4E's later versions of skill challenges and especially Apocalypse World do.You're right, of course, but there is another angle here. More codified systems create a barrier to entry for DMing. Yes it helps, but its a lot to digest for new DMs. In addition, more codified systems can "hide" bad DMs from themselves and players. Its hard to improve if you don't know whats broken. With more immediate, personal, feedback, DM skills can improve quickly, even for novices.