Without a minimum set of algorithms, you cannot have two protocols that
both use CMS talk to each other. This was a strongly-desired feature of
CMS, since the IETF strives to have its protocols work together, or at
least not clash in an ugly fashion.
The required set of algorithms is tiny. It assures us that if some protocol
developer comes along tomorrow and says, "hmmmm, CMS looks good" that they
don't pick some odd encryption algorithm that will never interact with
S/MIME or other CMS-using protocols.
--Paul Hoffman, Director
--Internet Mail Consortium