[racket-dev] `cond' / `when' / `unless' / etc bodies

We already have begin-with-definitions. Would there be a great penalty to
simply wrap every body-like sequence of expressions and definitions with
begin-with-definitions? The latter even allows more freedom than (let () def
... expr ...), for it allows (begin-with-definitions def-or-expr ...). I am
not sure whether or how this would conflict with optimization.
Jos