He diudn't make them better in the slightest, he made them look stupid. He told them, to their faces repeatedly that he was going to do everything he could to help their enemies kill them because he's evil and hates them and time and again they do nothing about it. His continued pressence is the single worst thing to happen to the show. Anya's acceptance into the group didn't reflect on them well either but at least she was made human and after Dopplegangland was pretty much done with evil. Spike? Spike was only around because he was popular. He wasn't there because of the plot or because they needed him he was only there for ratings. Every single character is dumbed down and made horribly out of character to let Spike get away with everything when they should, and if they'd been acting normally would, have finished him off.
Like I said, it's not that he's not an enjoyable character, he is, it's that they kept him around in a stupid, clumsy manner which made the rest of the cast look foolish.