He's an awesome dancer.
He's very interesting as a person. Most of his philosophy is nought but crackpot ramblings, ramblings which probably make him the funniest serial killer of recent times.
But a lot of people fell for that philosophy. A lot of people still do. In that way, he's more dangerous than the batman killer, more dangerous than the norway shooter, more dangerous even than a lot of fictional serial killers. Even in jail, he can try to convince people he's right.
He's a testament to the power of simply being able to manipulate people. Proof that you can take a perfectly sane man, and, with a few pushes in the right direction, make them do insane things.
We, as a society must never forget that, and be wary of others who can make that sort of impact on people, from lowly conmen to political leaders to Manson himself.