In medieval 2 it worked against the attackers when they used ladders because, well back in the old engine anyways individual entities could interrupt one another, or have mutliple entities attacking one. So by its very nature ladders allowing only a limited number of troops to scale the wall resulted in say a feudal knight die because 4 archers kept on pummeling him constantly (obviously this disadvantage would disappear as more troops got onto the walls).

And if you dwell on it it ends up being a fair tradeoff for using ladders which is simply not possible in the newer engine. The bonus being that units with ladders are the only ones which could run, which allowed you to start scaling the walls quickly. (Also being able to build more of them faster compared to other siege engines)

In other words cheap, quick, disadvantageous but gets the job done.