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.
See Tom's earlier point about the field you put the most troops in becoming your new center for the first point. As for the second, sure, you could miscalculate, but you're going to have at least a third of your force in the center, and if you are doing something like this, you're going to pick troops with max armour in box formation to hold out as long as possible. Five bucks says my center holds out longer than whatever crap you put on the flanks, even with the extra waiting rounds, and then you're getting hit with most of my troops with a sizable bonus.
Yes, the numbers on the bonus and waiting rounds would have to be tweaked, but for anything with a real chance to finally introduce serious tactics, chance, and kill blobs at the same time, I imagine the coders would be delighted to do that tweaking if the system were implemented.
The problem, as I said, is movement. If the flanks can get around the frontlines, where to they charge? The infantry to break up the shieldwall? Or do they carry on to hit the archers in the background? It all depends on many, many factors.
Why make it complicated? Have them show up in normal starting position with a "flanking bonus" to morale and cohesion or what have you that makes them significantly more effective fighters.
