We've all been in this situation, you killed a tough boss (or made it easy thanks to the zerg train); and what do you get as a drop? Some whites, a soul gem, and five potions. That's not very rewarding IMO. Some people don't have the time to sit at his spawn for 20 minutes to get one blue drop. What if bosses just had diminishing returns?
Ex) You kill him the first time, and depending on your zone/the bosses level, you get a good drop. As in Blue, Purple, etc. Next time you kill him, the gear isn't as good as the previous kill, so on so forth until it's white gear.
You've got the good loot you desired, and there is no more reason to farm him, aside from maybe soul gems and potions if you truly do desire. You can still farm him for experience, it's just that you won't be getting a solid stream of good loot if you sit there long enough.