I think the answer is the total guileification of the (extended) GNU operating system. We need to facilitate cross-talk between bits of the operating system, and there would be no more natural way to do this in GNU than to make all commands understand literal Scheme lists of arguments, and give back literal Scheme lists of data (including the response to '(help), which would make the entire system introspective!), so that inter-operation between sub-systems becomes natural.
As a Guile hacker, I also think that more programs should be extensibile via Guile. However, I don't think programs need to "understand" sexps in the way you're describing. Programs can continue to work as they have, taking in textual input and outputting textual output. What you get with Guile is the ability to use the program's API via the REPL so you can interact with and extend that program, much like Emacs.
