I've often wondered if they would have gone down the Willow's addiction route in Season 6 if Oz had still been around. It seemed like something they did to drive a wedge between her and Tara, to emphasize Tara's lessening interest in using powers and Willow's lust for power after healing Tara's mind in Season 5.

I would have loved to have seen Oz help Willow through her addiction.