I wonder where you got the idea that the rationale for this proposal is “because people are occasionally confused”. The actual rationale is much more reasonable:

    The primary problem with the "X-" convention is that non-standard
    parameters have a tendency to leak into the protected space of
    standard parameters (whether de jure or de facto), thus introducing
    the need for migration from the "X-" name to the standard name.
    Migration, in turn, introduces interoperability issues (and sometimes
    security issues) because older implementations will support only the
    "X-" name and newer implementations might support only the standard
    name.  To preserve interoperability, newer implementations simply
    support the "X-" name forever, which means that the non-standard name
    has become a de facto standard (thus obviating the need for
    segregation of the name space into "standard" and "non-standard"
    areas in the first place).

Also there is nothing in this proposal that would “effectively [double] the number of permutations of names they have to try”. Look at the Recommendations for Protocol Designers in section 4.