Actually, in the CHICKEN Scheme community, this is done quite often, though you don't see it nearly as much because of how transparent it ends up being. Mostly stuff is written in Scheme / R5RS when possible, and C bindings to a faster library or implementation are exported using the bind egg when speed is of critical importance.
This is obviously not the norm, as the bind egg is very good, but it's definitely out there. Makes me wish that there were other languages which offered something similar to bind, but alas CHICKEN is the only one I know of thus far.
This is obviously not the norm, as the bind egg is very good, but it's definitely out there. Makes me wish that there were other languages which offered something similar to bind, but alas CHICKEN is the only one I know of thus far.