all, now I get it, I saw the "under your control" part and wasn't understanding, lol.
Yeah, 24 hours of control is plenty of time to make sure the 8000 skeletons get where they need to go to start destroying the right things. You can use that time to smash the city's defenses and crush its main army. A normal person would freak out about what the e.g. 5000 remaining skeletons are going to do after that and how you're going to prevent them from killing innocent people tomorrow when you lose control, but the BBEG just doesn't care since all he wants is revenge/power/blood.