If the story had ended at Season Five, regardless of who the villain was, it would've sucked majorly.
Spike wouldn't have gotten his awesome chance to play the hero for once, nor would he have been able to play the mentally disturbed vampire who had actually sought out a soul, who'd been torn apart by his love for the slayer, which was awesome for me to watch, since Spike is my favourite character, and to watch him change so radically was great, it was a change, not to mention James is an awesome actor.
Plus, we wouldn't have gotten to see Evil Willow, or the Turok-Han, or Caleb, or any of those great villains and storylines.
Glory was a great villain and I enjoyed watching her, but it just wouldn't have cut it, in my opinion, as an end to the phenomenal show that Buffy is.