Ill give you my understanding real quick before someone else chimes in.
Normally, hubs are selectable (lock and unlock) either manually or automatically. Slugs basically just work like hubs that are locked all the time (ie, they can't be unlocked). Slugs would work if you had a limited slip or selectable locker, but wouldnt' be good with a fulltime locker or on teh street at all. Damn near indestructible though.