To my mind, the Employer has to have some way to update the plan for ongoing law changes and resulting changes in the LRMs. More, some parts of EPCRS required reliance on a favorable letter, and after the IRS has published a required amendment, the previous letter (even without an expiration date) may not satisfy.
Seems like a pre-approved document would be a no-brainer: relatively inexpensive with great reliance. I think the better question is "what alternative is better?" Maintaining the IDP long term likely isn't cost effective.