You put your cavalry, etc on the flanks and a weak holding force in the centre, as you said. But what happens if blob guy puts all his forces on that flank too? Your forces get crushed. What happens if you miscalculate and your centre loses faster than you think? You take rout damage (if that's implemented) and did nothing to hurt them. And I don't know how much your throwaway nobles will appreciate having been thrown away.
Their largest force is automatically center so their main force being in your flanks isn't possible.
