I have my own Scheme implementation (Gauche) and I live on writing software in it, and I keep trying new ideas with it. For that regards, I don't need Arc. But that kind of atmosphere at arclanguage.org, that's not something I can get without Arc.
Maybe most of the ideas tried there will turn out not to work, but new ideas need a place like there to come out. In a sense, Arc isn't a new programming language, but a medium that sheds a new light to the language design process.
And I'm watching it closely to steal whatever good ideas that come out :-)
I dunno. I could certainly be wrong - I was about Reddit. (Though I was also wrong about Xobni, and they did basically the opposite of everything PG suggests in this article.)