Umm, if I remember correctly, two of the reasons we were allowed season 5 were that James Marsters be brought on board and that the show didn't rely on an arc because the network wanted more people to come on board - because they lost viewers in season 4.
As for saying that Spike being '85%' of the funny of 'Angel' well um, can I call that a very lame comment.
Cordelia was pretty much the humour of Angel particularly the first couple of seasons, why aren't you complaining that she was '85%' of the humour. Same with Wesley?
Its not even like Spike was a new character. He was a well established character in the verse already, we knew what to expect from him AND most of his 'funny' was when he was WITH Angel, giving Angel some of the credit.
Season 5 has its issues. I didn't like Cautionary Tale, and I felt that 'Why we Fight' should have been done earlier in the season BUT I have to admit, I was quite happy that the show was less arc-y. Took it back to its roots, or do you forget that season 1 was this kind of show?
Angel, the character, was not supposed to be the comic relief, the shows established comic relief had left and in a show as dark as Angel can be, you need humour, you need light so I am thankful that Spike provided it because IF Angel didn't have Spike in season 5 AND we take the official story that CC wasn't going to be in season 5 anyway then we would have had an entirely new character to fill the 'comic' gap and I prefered Spike.