But how do you control the individual lines or columns after that?? For example, you may want your skirmishers or cavalry to move to the enemy's rear once the army is engaged, but with the formation (as one single group of all units) you can't do that easily....
Also, in my experience a static pre-battle formation never works, because AI knows exactly how to deal with it: for instance, AI would not send units into the gaps in your formation (which makes a maniple useless), instead it'd keep the first line/lines inact by minimal units, and put the other units in reserve or try to flank you - and make your flank-protection useless, unless you micro-manage them. In the end, the only thing that matters is the actions you take during the battle, not before. And that's why I always organize my army into 3-5 groups, each of them in a single-line, so I can micro-manage them easily.
Am I playing RTR battle in some very different way than other players?
(I'm using RTW-Alex, BTW)