We definitely need solidity in front of the defence, which in turn should promote some fluidity in midfield. However, formation at the back has been very important and has influenced many games. This is because how the backing sets up also influences play beyond defending. I've made no secret of my belief that Rio should wide right midfield with a roving mandate. He couldn't do that in a flat back four. Also changing formation can change the whole game. So a fluidity of formation probably remains the best approach.
I do agree with Lukcour in the sense that a lot of goals we concede are not linked to formation but an age old thing of standing off and giving opposition forwards time to make decisions.