I do know someone who's faked having AS. She's admitted this a few years after doing so.
In her case, she faked it because it was easier than coping with having been bullied and neglected. She heard about Asperger's, it matched some of the symptoms she'd developed as a result of how she'd been treated (and she was already introverted before that), and she started trying to make herself into that image because that was easier on her than coping with her past.
She did eventually get therapy, go on medication for anxiety and depression, and mostly deal with it, and then say that no, she's not autistic, she just is not normal and does have issues that include some similar to people on the spectrum.
However, it comes down to, its sometimes easier to have a reason to be the way you are, even if its not accurate, than to cope with being the way you are without the reason of being autistic.
I really don't think that there are many people who fake though. There are people who think they have it and are sub-clinical, but I don't think that's even as large as people tend to assume.