What I've noticed after using paredit is that I'm doing a lot less paren matching (well none) and a lot more white space clean up, so I think such a function would be very useful to run as a hook on insert char. Since the program is always syntactically correct, it might almost work...
And @sedachv, thanks for Parenscript. It makes Javascript almost...well, now I can work it it :) Much appreciated.
That would not be a good default because some people, like me, like to align their code in columns (for example, things like let clauses).
What I would like to see come as an Emacs default is just-one-space working on regions with the behavior you describe.