There's a crucial difference between obscenity and the other three categories, though. The other three are not protected because of the harms that result from them. Obscenity's lack of protection has nothing to do with any such harms; it simply isn't held to be *worth* the Constitution's protection. So it's easy enough for those of us who think the Supreme Court is wrong on this issue to do so without worrying about any slippery slope toward legalising solicitation of murder. Obscene speech could easily be protected with the exception for harm-causing speech kept intact.
When was the last time the Supremes held a work to be obscene, anyway?
Out of curiosity.