I do not comprehend.
EDIT: Granted, what you actually use either type for just varies. The mult thing is explained essentially because basic physicals always run off ATK, so you have a steady source of offense for trash mobs on your physicallers (if mage physicals ran off MAG instead... sigh). Against bosses, the physical skills serve utility purposes rather than raw damage. In the early game, this doesn't shape up quite that way because L1 and L2 magic is trashy and physical skills frontload their offense, but at this power level that's just how it goes. The gap is likely to widen as rarity-3 craft items become more of a commodity rather than a premium (Heroic daily dungeons drop those like candy). At some point, I imagine the ideal party shifts from three fighters+one mage+one healer to at least two mages+two fighters+a healer, because honing L3s becomes progressively easier and they actually last you even against boss rush stats, while physicals get offensively outdated even with 5-rarity material, which come at a serious premium.