Are you sure your MATS does not provide more guidelines for priorities? Here in New Zealand, our MATS (based on ICAO standards), more or less states the priorities you listed, then goes on to say the rest is on a first come, first served basis, with 10 exceptions. The exceptions are more or less common sense, with one being (can't remember the
exact wording):
Where a significantly greater economic penalty would occur by allowing an aircraft to operate ahead of another. e.g. allowing a light aircraft to operate ahead of a jet.
Though this can be an annoyance for light pilots at aerodromes operating scheduled jets, it does make sense. A single or even twin engine piston would have to be at the holding point far longer to consume as much fuel as a Boeing would in a few minutes. Additionally, if the Boeing were to miss it's slot, there is potential for further expensive in-flight delays.