Orius
Unrepentant DM Supremacist
I too see it as more of a guideline. I think it's presented as more of a hard and fast rule for the benefit of less experienced DMs who are more likely to go by the book. There's also the matter of XPs being based on XPs, I think discouraging the use of ELs higher than the party might be there to prevent characters from leveling too quickly.
It might be a hard and fast rule, but good and experienced DMs aren't necessarily going to follow them because they know how to set up an encounter for their players. Also, XP gain isn't necessarily thrown out of whack either because the DM can resort to ad hoc or story XP awards rather than just going by the book on XPs. BTB XPs are there again for the benefit of less experienced DMs who don't know how to award XPs. Many experienced DMs never go BTB on XPs anyway, and I think the DMG sort of reflects that with ad hoc and story XPs.
It might be a hard and fast rule, but good and experienced DMs aren't necessarily going to follow them because they know how to set up an encounter for their players. Also, XP gain isn't necessarily thrown out of whack either because the DM can resort to ad hoc or story XP awards rather than just going by the book on XPs. BTB XPs are there again for the benefit of less experienced DMs who don't know how to award XPs. Many experienced DMs never go BTB on XPs anyway, and I think the DMG sort of reflects that with ad hoc and story XPs.