Is there a 'realistic' use for mercenaries?

[LorDBulA]

If I have a choice I dont use mercenaries for suicidal missions.
They are very expensive to replace but are not expensive to upkeep.
From financial point of view it doesnt make sense to hire mercenaries and annihilate them in first battle.
That said it had happened few times. But I just didnt have a choice.
You just cant assault city with hors army.

[Sarkiss]

If I have a choice I dont use mercenaries for suicidal missions.
They are very expensive to replace but are not expensive to upkeep.
From financial point of view it doesnt make sense to hire mercenaries and annihilate them in first battle.
That said it had happened few times. But I just didnt have a choice.
You just cant assault city with horse army.

agreed.
in any battle i fight i throw them in only if absolutely nessecary, otherwise they are kept as reserves (in most cases).
main reasons they are so valuble to me -
1. they are very expensive to hire anew.
2. you cant retrain them in most cases.
3. like said above, they can fill the gap in what otherwise just isnt your faction's strong point.