We have a 30+ year history that an inadequately specified "priority" field
will go unused.
Actually we have exactly the opposite. We have at least two inadequately
specified QoS mechanisms for email already (priority and precedence header
fields), and both of them are in wide use.
I suspect the point is not really that they will go *entirely* unused, but
that a poorly specified priority mechanism will not be very useful overall,
and so will not be *widely* used in comparison to the scope of Internet
mail. Stated that way, I'd have to agree.
Keith