+1 to can't use because it is heavy armour and Skaven can't use heavy armour.
Although he can't use it, one reason for keeping it and not selling it is for the purposes of trading. For example, if one of your guys was captured your opponent want to kill your guy so he can keep the warplock pistols. He wouldn't be tempted by mere gold, but he *might* be tempted by a suit of gromril armour. Alternatively it could be used to sweeten a ransom deal where *you* have the prisoner - e.g. "I will give you your Youngblood and this suit of Gromril if you give me that Tome of Magic" or something.
For what it is worth with the armour, in my group we have house-ruled that Ithilmar is treated as Light armour rather than Heavy.