Bob, W9DMK wrote:
"-maybe the rationale is simply to decide whether or not a particular
sample of material is a "high loss" or not."

I think that`s it. The properties of dielectrics are functions of
temperature and frequency. My CRC Handbook of Chemistry and Physics
shows PVC`s dielectric constant is definitely a function of frequency,
unlike polystyrene and some other pastics.

I`ve seen PVC used as a base insulator for vertical elements of a HF
antenna array, and it worked OK.

I think low power factor and dissipation=A0are desirable for an
insulator. My 1987 edition of the ARRL Handbook has a table 47 on pages
35-36 and 35-37 titled "Properties of Common Thermoplastics. Maybe later
editions have expanded tables.

Best regards, Richard Harrison, KB5WZI