I don't understand two things:
A : Do people just not know what end means? Because there seems to be some confusion.
B : What, exactly, is the practical difference between playing from the final save and playing after the ending slide? Seriously?

We saw in OWB the effects of mixing ending slides and playing AFTER the ending. To nobody's surprise, it's just goofy and invalidates anything the slides said. Let's not make the same mistakes in the core game.

But I wanna wait two in-game weeks for my Imperial Dragon Armor.
http://www.uesp.net/wiki/Oblivion:Imperial_Dragon_Armor

Nah, that was pre-order DLC... now avilable for the low cost of 400 MS-Points.

As long as it's common knowledge that the end is the END, I see no problem. IMO that adds replay value. I have yet to finish me initial playthrough (which may or may not be related to a deathclaw-owned hissy fit and subsequent distraction with other games), but I can't wait to join the legion, and then dive in a 3rd time to help Monsieur house.

I'm also dicing with whether to stick with hardcoe mode or not to speed up subsequent playthroughs. hardcoe mode was a masterstroke from obsidian and I'm disappointed bethesda didn't cherrypick that idea for skyrim.

A giant text box leaps up telling you that this is the point of no return, and if you want to finish up other quests you should make a save before you play on.

And again, what's the difference between playing after the credits and playing after that box pops up?

This is why Dead Money is considered such a well done DLC, once your done YOUR DONE. It adds a sense of accomplishment, for me anyway.
IMO, Obsidian did their best to deliver a conclusive ending, and being able to play around after the "end" simply dwarfs/ takes away from the experience, aswell as making their work seem.... Meh.