It is not harmful either. But it helps to detect if an implementation of
the R7RS library is broken in the sense that it does not allow
interdependent libraries. If a program imports as follows (import (srfi
128) (srfi 146)), it would be an error (detectable at expansion time) if
the Scheme system loaded (srfi 128) twice, once for importing into the
program, and once for importing into (srfi 146). Namely, in that case
the main program would import comparator? with two different bindings.
I think, at least logically, the SRFI says exactly what you want it to say.