It’s still perfectly possible to use all combinations, including small/small, and it rides fine. The chain runs a bit slack but the mech guides it onto the cassette ok.
I just prefer to have as little slack chain as possible flapping around on the fast n bumpy stuff, with the intention of reducing the risk of the chain coming off. Some may prefer to run a long mech to tame a longer and potentially ‘flappier’ chain.
Each to his/her own, if we were all the same it’d be a boring world!
Rob