There was a great interview with Bruxe Tate on the "Mostly Erlang" podcast some time last year. He discusses the decision process behind the languages that were included in the original "Seven Languages" book. From what I gathered, the reason Io was included was because Javascript is simply too complicated and multi-paradigm. The syntax and reasoning behind Io is extremely simply and illustrates prototypal principles beautifully.

It's never occurred to me to use Io for anything, but it's a wonderful mind expander.