Useless? I wish they were useless. Joyce was useless, Dawn was useless, except for that one time when she stopped Buffy from killing herself, again, hell, Spike was useless, he did nothing to stop the trio of anti-intellectual stereotypes or Willow on drugs. Useless is not so bad. Giles, Xander and Willow in season 7 are not useless, they are irrelevant. It's just not their story anymore. What do they even do?
Giles is, eh, waste of keyboard.
Xander tries to find a new girlfriend, goes back to Anya, looses his eye, looses Anya. That's it. Also, he fixes crap. Wait, is this Xander freaking Harris we're talking about or just some random wanker? Xander is less important in season 7 than both Andrew and Robin. Seriously, look at the plot, Xander does nothing of relevance, nothing! Erase him from the script, nothing changes.
Willow. Disclaimer: I hate season 7 Willow. I hate her. It's not dislike, not antipathy, no, it's pure, undiluted hatred.
Alright, what does Willow do in season 7? Whatever Buffy tells her to do. That's Willow's entire everything in this season, she makes zero plot-relevant decisions. Nothing. You could swap her with just about any random witch, Amy or even Anya. No, seriously. This is the final season, the end of the story, it's fair to expect a conclusion, an all out, guns blazing, ass kicking hell of an ending for each of the core characters. And that's what I expected, I expected Willow to oh I don't know, marry zombie/robot Tara, turn Amy into a rat and feed her to Miss Kitty II or at least to finally woman up and stop being Buffy's pathetic, reliable, meek little minion. Challenge her, sleep with her boyfriend just for shit and giggles, kick her super-ass or die trying. Whatever, I'd take everything.
No, all I got was dark side is bad kids, freedom is wrong bullshit and the most boring love interest ever. So irrelevant. This is Willow we're talking about here, the second most important character - amount of dialog lines, scenes, episodes that focus on her - she's second to Buffy only and she could as well be swapped with a random, generic, place holder character, the plot would not change. It's ridiculous.