On the whole cycling thing... bacteria needs its food brought to it. It'll grow on whatever, but if there's no water movement only a minimal amount of bacteria is going to be able to survive.
Plants are somewhat the same way... movement usually makes for better growth.
So a filter might not technically be necessary, but it (or anything else that causes movement) helps for reasons beyond filtration.
No filters in nature, but stagnant water is pretty gross in nature too.
Anyway, moot point now.