call/cc による unlimited continuation だけではなく、delimited continuation が扱えるように shift と reset を R7RS small に入れたら良かったのでは、という疑問を持ちました。実際"An argument against call/cc"に説明されているような積極的な根拠があります。
その答えは "Scheme Reports Working Group 1 Progress 2012"とされる文書に書かれていました:
Some interesting comments from the community were simply beyond the scope of our standard. Oleg Kiselyov made a detailed and persuasive case against undelimited continuations as in call/cc in favor of delimited continuations as in shift and reset. Unfor- tunately, though delimited continuations are not by any means a new concept, they have relatively little support among implementations in comparison to call/cc. Moreover, our backwards-compatibility requirements make it impossible to remove such a fundamental part of the language. What we can and will do, however, is provide delimited continuations in addition to call/cc in the large language, and hope their support and use increases so this can be revisited in later standards.
というわけで R7RS large に期待しましょう。