This makes sense - what about people who advocate putting all of your army in the middle column, is there any merit to that?
The main problem is flanking bonus. 30% may look not that much but they really may turn the battle in enemy favour.
It depends on a number of factors, such as troop composition, available generals, expected enemy and terrain bonuses. First and pretty important for minor lords factor is this: you need only one skilled general. It may be hard to find three equally skilled generals, so if you are in that situation, do not hesitate and unite flanks in one.
Second, you have to consider how well will your troops fare in skirmish. If you are using heavy troops, making amphibious assault to mountains or for some other reason don't expect even the united troop to break one third of enemy troop before melee ensues, you are generally better off uniting your troops, because losses (and morale hits) in melee every day are tremendous, while it would take couple of days first to finally break opposing flank, and afterwards it will take several days before pursue is switched by melee with another flank, all these days your troops are decimated without hitting in return. If, on the other hand, you expect to crush the enemy in skirmish, then united flank is pretty viable.
Third, know your enemy. AI tends to distribute troops evenly, but big stacks may break this distribution. Big stacks may be mercs, or, worse, holy orders. If your opponent employs holy orders while your troops are in single flank, you either have to guess on which flank that steel fist would be, or get massacred. Seriously, don't gamble with holy orders. If they flank you, you may not be able even to take a single soldier alive out of that battle.
And lastly, but not the least, predictability. There are better tactics and there are worse. If you pack all your troops in one flank, you gamble. All of your troops may heroically charge and smash the enemy, or they all will get confused orders and get wiped out. While with even distribution of troops the extremities are less possible, downfall of one flank would be compensated by success of another. However, if you distribute troops asymmetrically, you may even inflate which flank would tend to use which tactic, because tactic probabilities depend on troop composition (i.e. with over 50% archers you are more likely to use volley, while with over 40% HI and\or over 30% of archers you are more likely to use feint). And as most tactics that get their chance increased grant bonuses mostly to the required troop type, while often granting penalties to other troops types, you troops get overall more effective (compare two mixed infantry-cavalry flanks, that may either improve infantry half of the flank or cavalry half, with mostly HI flank having increased chances to get HI tactics and mostly HC flank having increased chance to get fully empowered with charge).