Attacking a castle isn't easy enough guys ? you want to add yet another tool for offense that wins 99% of the time already ?
Well, I think you are not seeing the big picture. If you have a bigger army than the garrison is normal that you can win the fight, its "real", however, the lacks in some features, "easy to implement" that can make it more realistic and challenging its the problem here (its what i think).
For example, they can implement a possibility of disease in both armies which is something normal in real sieges, (they can parameterize it using your leadership, tactics and medic skills + the army size + the time of siege). This can add a plus of difficult to novice generals or to castle without a proper governor. Another example is the possibility to send a companion to do a subterfuge action, like poison the well, burn the stables, burn the granary (if attacker), or burn the stable, burn the tents, assassinate general (if defender) (companion should need the roguery skill). These are just some ideas that only need some dialog and logic, not scenes, not nothing fancy, but they will give you more gameplay and will make skills more useful.