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.

It's interesting to consider the parallels between this and CSS vendor prefixes.