To address the issues raised last post, R’ Elchanan poses a chakira: why is it that issurei hana’ah have no value – is it a din or a metziyus? Meaning, is value simply a practical function of whether or not something can be used, or is it a halachic / legal function of whether it is permissible to use an object? If value is simply a function of practical use, it makes no difference whether an item is assur b’hana’ah mederabbanan or m’doraysa. However, if value is a function of the halachic right to use an item, then an item may on a d’oraysa level be classified as having value, but mederabbanan be off limits and valueless.
If value is a function of the halachic right to use an object, it becomes easier to understand why Rashi (Pesachin 7) resorts to the mechanism of “afk’inu rabbanan l’kiddushin” to explain why kiddushin using chameitz derabbanan is invalid. Practically, chameitz which is assur b’hana’ah mederabbanan is useless and valueless. However, legally, halachically, that chameitz on a d’oraysa level is permissible to use and does have value. Rashi defines value based on the legal rights to use an object. If m’doraysa an object has use, it has value, and m’doraysa the kiddushin is valid. Rashi therefore introduces “afk’inhu l’kiddushin” to explain how the Rabbanan can dissolve kiddushin which is effective on a d’oraysa level.
R' Elchanan writes that this approach also explains why according to the KS”M (and some Rishonim) kiddushin may work using a double derabbanan (chameitz derabbanan during a time period where the issur applies only mederabbanan). Since value is measured by halachic rights, there is room to distinguish between an item excluded from use by one derabbanan vs. two.
For more on this issue, see R’ Shimon Shkop in Sha’arei Yosher I:9-10.