What is better: A single report from a vehicle with unknown tires and brakes, or a PIREP by an airplane of a type known to you? How can YOU translate a contaminant type & depth to a braking assessment for YOUR airplane?
Though I fly the 744, I suspect that many heavy/large (and even smaller) transport airplane types with antiskid systems behave similarly. Whether or not a particular pilot on a particular type may have the "feel" of the brakes is yet another question. Still, I can feel when the antiskid system releases the brakes, whether using autobrakes or manual braking. I can do a subjective assessment of how often and for how long the antiskid releases the brakes, also knowing the level of effort (autobrake setting or foot pressure). The more the antiskid has to work, and the more I have to compensate with rudder for asymmetry, the worse the condition is. In a nutshell:
No antiskid releases, or infrequent, short antiskid releases: Good
Noticeable, but controlled, antiskid releases: Medium/Fair
Frequent, prolonged antiskid releases and/or large asymmetric releases: Poor.
For those without antiskid systems, translate those releases to the requirement to manually release or moderate brake pressure to a degree less than otherwise desired.
In any case, such a PIREP system translates to a performance level that another pilot can relate to, even if not "exact" for his airplane type. After all, NO performance calculation on a contaminated runway can ever be "exact".