No need for Bethesda to waste development time on this nonsense.
One word: Fraps.
Stop with this argument
please. Bethesda is a very very large company now if you haven't noticed, if they can't add things like this without it impacting on other parts of development then they need to fire their project managers.
Edit: There is a difference between just recording a video of whats happening, and recording data like player and enemy locations and movements in order to re-render them again later in the demo viewer.
They already have the tools to record world states and replay them later, because they'll use them for trailers and what not.