Dear gods does mithral do a lot for armor
Lightweight (grade 1), mastercraft III (reduces armor check penalties by 3, grade 3), increases max Dex bonus to AC by 2 (call it grade 2 for now), decreases arcane spell failure by 10% (twilight quality from Magic Item Compendium
was a +1 armor special ability, which maps to grade 2 here), and armor is treated as one category lighter (heavy -> medium; I consider this a grade 1 quality).
Nine grades of benefits. A +4 armor enhancement is a grade 8 quality. It's not eligible in a grade 9 item, but that's the size of the total effect.
That's a lot
to make available at the low price presented in the PRD. Unless you want something specific from another material, I have difficulty understanding why basically all magic armor isn't mithral.http://www.kjd-imc.org/blog/weapons-and-armor-made-of-special-materials/