Things You Never Noticed in 2001: A Space Odyssey

1) There might be a power or mechanical failure that prevents the ring section from turning, and you don't want to be stuck years from Earth with most of your food supply inedible in zero-G.

2) "Caviar is cheaper than Cornflakes in space." The paste food would have a much higher nutrition-to-weight ratio. A Hungry-Man dinner wouldn't be able to justify the fuel cost to put it into Earth orbit, let alone a mission to Jupiter.

Also, triggering the explosive bolts on the Pod probably should be easier than using a Zero-G toilet; you wouldn't care where the debris from the bolts end up.