I could see Faith's character having her fan base diminish by leaps and bounds if she didn't go evil. Although we (I'm assuming here) all wanted the Scoobies to do a group hug with Faith and stroke her hair saying "Oh honey, it's ok" instead of using Faith like a tool the entire season, look at how much better the show is with Faith going evil.
Speaking for myself, ever time i see the episode where Angel fakes being turned into Angelus by Faith/Mayor, I'm always wanting both of them to go evil and do stuff.
A good example of what i'm saying could be taken from Vampire Diaries. I believe that the characters who were good but went evil for a time, made the show much more attractive/must see. (Caroline, i'm looking at you girlfriend. I remember pointing at my TV going 'It's blonde Faith!')