I've heard explanations for why Amber Benson didn't come back, but I think it would have been so much better if she had. Having Azura Skye, while she is quite awesome, it was just like....you know, I was with Willow, I didn't get WHY. They didn't come up with a good enough excuse for her absence, it was just like "Well...she CAN'T, okay? God, get over it."
IMO, possibly the worst thing about Willow's section is that they couldn't come up with any in-story reason why Cassie was there and not Tara. It just further added to the incomprehensible nonsense that was the First's plan.