Are there any situations where the splicing versions could not serve as
a drop-in replacement for the non-splicing ones?

In another thread, Marc said:

> The non-splicing binding constructs `let-syntax' and `letrec-syntax' as provided by R7RS are somewhat superfluous because they give the macro programmer nothing that cannot be expressed with `define-syntax'.

> From a theoretical point of view, there is no reason why binding constructs for syntactic keywords should not be splicing, that is, why they should not have the same semantics as `begin'.