Well I guess there was no choice but to make Xander fat since Nicholas Brendan was fat...I remember one late episode where he was in bed with Anya and it looked ridiculous since Emma Caulfield was in her annorexic phase. But yearh, there's no excuse for how they made Xander so useless. The last Xander centric episode was right at the start of season five (The Replacement)!
The only good stuff he did in season 7 was his speech to Dawn about her being special and his speech about Buffy before he lost his eye (season 7 had a lot of speeches.)
The problem with Spike is that he became so popular they had to keep having him back and making him cooler and come up with reasons why Buffy didn't just kill him so eventually he just became a good guy. I like season four Spike the most where he's still evil but works with the Scoobie because he enjoys violence.