Why should local bindings use let? In scheme internal (define ...) is defined in terms of letrec, which can add some overhead when letrec is unoptimized, but for schemes with optimized letrec (guile 3.0, chez, racket...) (define ...) is just as fast as let or let* unless you start doing cyclic references.

The only real reason to not use Def would be if the bound symbol escapes the scope, but that would be ridiculous for a modern language.