There are some games that have 21:9 cutscenes by default on a 21:9 monitor, Arkham Origins comes to mind, as does Wolfenstein.
Others are usually modded via Flawless widescreen to allow for this. Dragon age Inquisition included. The results are epic.
I can see why developers might shy away from testing everything on a 21:9 as it would add to the workload, but in most cases all that is needed is to remove the black bars, the framerate usually stays the same as the op stated the scene is usually still being rendered behind the bars.
In all the games I've played like this I can only think of a couple of examples where I saw something I probably shouldn't have. I'd much rather have the option so it looks right for the rest of the time.
