Summoner virus reduces str, dex, int, and mnd by 15%. Whm/blm only str and dex. Summoner virus still better. What you are discussing is content design flaws. Not summoners fault that Square Enix made the damage types physical or unmitigable, and SCoB turn 2 final add is caster type. Also from a lore point there is no reason why Arcanist/summoner can't use scathe, sleep or manaward, which are thaumaturge skills, not black mage. Job skills are not available for cross class, only the base class skills are.