Its one of the main points to further improve their use, i mean different armor bonus for each upgrade.
And i dont think MM will be getting such a summon, according to lore they didnt have anything to do with DG, so theres that:/
But i think this would be the best way to include the epic botfa trolls, a way that actually makes sense improving the gameplay.
But to make the point clearer.
Hammer with Morgul armor bonus vs arrows, cause they are used most when breaching a fortress and destroying buildings, and there will be alot of archers on the wall
Wooden mace with DG armor bonus vs spears, cause they are used in the open, where people often use spearmen to counter you.
Sword with Morannon armor, so you can fight single targets and heroes better.