In the last episodes casey transforms the spritesheets from the artist to contiguous bytes in the bitmap for the asset system. But he also mentioned that downloading assets to the graphics card should really be spritesheets instead single bitmaps. Isn't it better to update the assets system itself to work with these spritesheets so they can later be downloaded to the card in 1 go easily?

It will be more efficient, but since Casey doesn't have performance problems with the graphics it's no necessary at the moment. Since the current asset system works well he can keep it to continue developing the game and only change it when it's necessary (to avoid unnecessary work). The sprite sheet you get from the artist might need to be processed to be more tightly packed to save bandwidth and that's also some extra work.