I disagree (though the marching formations idea is pretty cool) not because it's an objectively bad idea, it's a good idea on its own. But the key is that Bannerlord has you taking the role of a general on the ground. Historically, you wouldn't have been able to get a bird's eye view of the battle and teleport your troops where you wanted them before battle was joined. When two armies met in the field, they had to deploy at that moment. There are quite a few major battles which were won precisely because one side wasn't able to deploy fast enough.
I like it for sieges because you'd have plenty of time to array your forces during a siege, whichever side you're on. But for field battles, you're either in a marching formation or camping when you contact the enemy, and in either case your men wouldn't already have had time to deploy. For that reason, I prefer the system as we have it. It forces you to see things from the perspective a general in history would, and that's classic Mount and Blade right there.