Bradley Lucier says:
I don't see the centered-* operators as somehow a "completion" of the other division operators. (2) In the small language I'd recommend only the truncate-* and floor-* operators for two reasons: they are the only division operators that have an established history of use in computer programming and mathematics, and they form a minimal extension of R5RS. (I'm not saying that the other division operators have never been used in mathematics or programming (see CL), but small Scheme is not supposed to be a kitchen-sink language.)
The WG decided to shrink the division procedures to just truncate-* and floor-* (plus the R5RS-compatible names) and place them in the (scheme base) module.